Editing Neptunium

{{Infobox neptunium}}
'''Neptunium''' is a [[chemical element]]; it has [[chemical symbol|symbol]] '''Np''' and [[atomic number]] 93. A [[radioactivity|radioactive]] [[actinide]] metal, neptunium is the first [[transuranic element]]. It is named after [[Neptune]], the planet beyond [[Uranus]] in the Solar System, which uranium is named after. A neptunium atom has 93 [[proton]]s and 93 electrons, of which seven are [[valence electron]]s. Neptunium metal is silvery and [[tarnish]]es when exposed to air. The element occurs in three [[allotrope|allotropic]] forms and it normally exhibits five [[oxidation state]]s, ranging from +3 to +7. Like all actinides, it is [[Radioactive decay|radioactive]], [[radiation poisoning|poisonous]], [[pyrophoricity|pyrophoric]], and capable of accumulating in [[bone]]s, which makes the handling of neptunium dangerous.

Although many false claims of its discovery were made over the years, the element was first synthesized by [[Edwin McMillan]] and [[Philip H. Abelson]] at the [[Berkeley Radiation Laboratory]] in 1940.<ref>{{Cite journal|last1=McMillan|first1=Edwin|last2=Abelson|first2=Philip Hauge|date=1940-06-15|title=Radioactive Element 93|journal=Physical Review|volume=57|issue=12|pages=1185–1186|doi=10.1103/PhysRev.57.1185.2|bibcode=1940PhRv...57.1185M|doi-access=free}}</ref> Since then, most neptunium has been and still is produced by [[neutron irradiation]] of uranium in nuclear reactors. The vast majority is generated as a by-product in conventional [[nuclear power]] reactors. While neptunium itself has no commercial uses at present, it is used as a precursor for the formation of [[plutonium-238]], which is in turn used  in [[radioisotope thermal generator]]s to provide electricity for [[spacecraft]]. Neptunium has also been used in [[neutron detector|detector]]s of high-energy [[neutron]]s.

The longest-lived [[isotope]] of neptunium, neptunium-237, is a by-product of [[nuclear reactor]]s and [[plutonium]] production. This isotope, and the isotope neptunium-239, are also found in trace amounts in [[uranium]] ores due to [[neutron capture|neutron capture reactions]] and [[beta decay]].<ref name="CRC">{{cite book| author = C. R. Hammond| title = The Elements, in Handbook of Chemistry and Physics| edition = 81st| publisher = CRC press| isbn = 978-0-8493-0485-9| date = 2004| url-access = registration| url = https://archive.org/details/crchandbookofche81lide}}</ref>
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==Characteristics==

===Physical===
Neptunium is a [[hardness (materials science)|hard]], silvery, [[ductility|ductile]], [[radioactivity|radioactive]] actinide [[metal]]. In the [[periodic table]], it is located to the right of the actinide [[uranium]], to the left of the actinide [[plutonium]] and below the [[lanthanide]] [[promethium]].<ref name="Yoshida718" /> Neptunium is a hard metal, having a bulk modulus of 118&nbsp;[[pascal (unit)|GPa]], comparable to that of [[manganese]].<ref>{{cite journal |last1=Dabos |first1=S. |last2=Dufour |first2=C. |last3=Benedict |first3=U. |last4=Pagès |first4=M. |date=1987 |title=Bulk modulus and P–V relationship up to 52&nbsp;GPa of neptunium metal at room temperature |journal=Journal of Magnetism and Magnetic Materials |volume=63–64 |pages=661–3 |doi=10.1016/0304-8853(87)90697-4|bibcode = 1987JMMM...63..661D }}</ref> Neptunium metal is similar to uranium in terms of physical workability. When exposed to air at normal temperatures, it forms a thin oxide layer. This reaction proceeds more rapidly as the temperature increases.<ref name="Yoshida718" /> Neptunium melts at {{Nowrap|639 ± 3 °C}}: this low [[melting point]], a property the metal shares with the neighboring element plutonium (which has melting point 639.4&nbsp;°C), is due to the [[orbital hybridization|hybridization]] of the 5f and 6d orbitals and the formation of directional bonds in the metal.<ref name="Yu. D. Tretyakov" /> The [[boiling point]] of neptunium is not empirically known and the usually given value of 4174&nbsp;°C is extrapolated from the [[vapor pressure]] of the element. If accurate, this would give neptunium the largest liquid range of any element (3535&nbsp;K passes between its [[Melting point|melting]] and [[boiling point]]s).<ref name="Yoshida718" /><ref name="Gray" />

Neptunium is found in at least three [[allotrope]]s.<ref name="CRC" /> Some claims of a fourth allotrope have been made, but they are so far not proven.<ref name="Yoshida718">Yoshida et al., p. 718.</ref> This multiplicity of allotropes is common among the [[actinide]]s. The [[crystal structure]]s of neptunium, [[protactinium]], uranium, and plutonium do not have clear analogs among the [[lanthanide]]s and are more similar to those of the 3d&nbsp;[[transition metal]]s.<ref name="Yu. D. Tretyakov">{{cite book|editor=Yu. D. Tretyakov|title = Non-organic chemistry in three volumes| place =Moscow|publisher = Academy|date = 2007|volume = 3|series = Chemistry of transition elements|isbn = 978-5-7695-2533-9}}</ref>

{| class="wikitable" style="margin:auto; text-align:center;"
|+ Allotropes of neptunium<ref name="Arblaster 2018" /><ref name="Yoshida718" /><ref name = "alo">{{cite journal | last1 = Lee | first1 = J. | last2 = Mardon | first2 = P. | last3 = Pearce | first3 = J. | last4 = Hall | first4 = R. | title = Some physical properties of neptunium metal II: A study of the allotropic transformations in neptunium | journal = Journal of Physics and Chemistry of Solids | volume = 11 | pages = 177–181 | date = 1959 | doi = 10.1016/0022-3697(59)90211-2 | issue = 3–4|bibcode = 1959JPCS...11..177L }}</ref>
|-
!Allotrope
!α (measured at 20&nbsp;°C)
!β (measured at 313&nbsp;°C)
!γ (measured at 600&nbsp;°C)
|-
!Transition temperature<ref name="Arblaster 2018" />
|(α→β) 280&nbsp;°C
|(β→γ) 576&nbsp;°C
|(γ→liquid) 639&nbsp;°C
|-
!Crystal structure
|[[Orthorhombic]]
|[[Tetragonal]]
|[[Body-centered cubic]]
|-
![[Pearson symbol]]
|oP8
|tP4
|cI2
|-
![[Space group]]
|''Pnma''
|''P42<sub>1</sub>2''
|''Im{{overline|3}}m''
|-
!Density (g/cm<sup>3</sup>, <sup>237</sup>Np)<ref name="Arblaster 2018" />
|20.45
|19.36
|18.08
|-
![[Lattice parameter]]s ([[picometer|pm]])<ref name="Arblaster 2018" />
|''a'' = 472.3<br/>''b'' = 488.7<br/>''c'' = 666.3
|''a'' = 489.5<br/>''c'' = 338.9
|''a'' = 351.8
|}

[[File:Phase diagram of neptunium (1975).png|thumb|upright=1.1|left|Phase diagram of neptunium]]
α-neptunium takes on an [[orthorhombic]] structure, resembling a highly distorted body-centered cubic structure.<ref name="Lemire">Lemire, R. J. et al.,''Chemical Thermodynamics of Neptunium and Plutonium'', Elsevier, Amsterdam, 2001.</ref><ref>{{cite web|url=http://cst-www.nrl.navy.mil/lattice/struk/a_c.html|title=Crystal Lattice Structures: The αNp (Ac) Structure|publisher=United States Naval Research Laboratory Center for Computational Materials Science|access-date=2013-10-16|url-status=dead|archive-url=https://web.archive.org/web/20121002050018/http://cst-www.nrl.navy.mil/lattice/struk/a_c.html|archive-date=2012-10-02}}</ref> Each neptunium atom is coordinated to four others and the Np–Np bond lengths are 260&nbsp;pm.<ref name="Yoshida719">Yoshida et al., p. 719.</ref> It is the densest of all the actinides and the fifth-densest of all naturally occurring elements, behind only [[rhenium]], [[platinum]], [[iridium]], and [[osmium]].<ref name="Gray">Theodore Gray. ''The Elements''. Page 215.</ref> α-neptunium has [[semimetal]]lic properties, such as strong [[covalent bond]]ing and a high [[electrical resistivity and conductivity|electrical resistivity]], and its metallic physical properties are closer to those of the [[metalloid]]s than the true metals. Some allotropes of the other actinides also exhibit similar behaviour, though to a lesser degree.<ref>Hindman J. C. 1968, "Neptunium", in C. A. Hampel (ed.), ''The encyclopedia of the chemical elements'', Reinhold, New York, pp. 434.</ref><ref>{{cite journal | last1 = Dunlap | first1 = B. D. | last2 = Brodsky | first2 = M. B. | last3 = Shenoy | first3 = G. K. | last4 = Kalvius | first4 = G. M. | date = 1970 | title = Hyperfine interactions and anisotropic lattice vibrations of <sup>237</sup>Np in α-Np metal | journal = Physical Review B | volume = 1 | issue = 1| pages = 44–46 | doi = 10.1103/PhysRevB.1.44 | bibcode = 1970PhRvB...1...44D }}</ref> The densities of different isotopes of neptunium in the alpha phase are expected to be observably different: α-<sup>235</sup>Np should have density 20.303&nbsp;g/cm<sup>3</sup>; α-<sup>236</sup>Np, density 20.389&nbsp;g/cm<sup>3</sup>; α-<sup>237</sup>Np, density 20.476&nbsp;g/cm<sup>3</sup>.<ref name="critical">{{cite web|publisher = Institut de Radioprotection et de Sûreté Nucléaire|title = Evaluation of nuclear criticality safety data and limits for actinides in transport|page = 15|url = http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf|access-date=2010-12-20 }}</ref>

β-neptunium takes on a distorted tetragonal close-packed structure. Four atoms of neptunium make up a unit cell, and the Np–Np bond lengths are 276&nbsp;pm.<ref name="Yoshida719" /> γ-neptunium has a [[body-centered cubic]] structure and has Np–Np bond length of 297&nbsp;pm. The γ form becomes less stable with increased pressure, though the melting point of neptunium also increases with pressure.<ref name="Yoshida719" /> The β-Np/γ-Np/liquid [[triple point]] occurs at 725&nbsp;°C and 3200&nbsp;[[pascal (unit)|MPa]].<ref name="Yoshida719" /><ref>{{cite journal |last1=Stephens |first1=D. R. |date=1966 |title=Phase diagram and compressibility of neptunium |journal=Journal of Physics |volume=27 |issue=8 |pages=1201–4 |doi=10.1016/0022-3697(66)90002-3|bibcode = 1966JPCS...27.1201S }}</ref>

====Alloys====
Due to the presence of valence 5f electrons, neptunium and its alloys exhibit a very interesting magnetic behavior, like many other actinides. These can range from the itinerant band-like character characteristic of the [[transition metal]]s to the local moment behavior typical of [[scandium]], [[yttrium]], and the [[lanthanide]]s. This stems from 5f-orbital hybridization with the orbitals of the metal [[ligand]]s, and the fact that the 5f orbital is [[relativistic effects|relativistically]] destabilized and extends outwards.<ref name="Yoshida720">Yoshida et al., pp. 719–20.</ref> For example, pure neptunium is [[paramagnetic]], Np[[aluminium|Al]]<sub>3</sub> is [[ferromagnetic]], Np[[germanium|Ge]]<sub>3</sub> has no magnetic ordering, and Np[[tin|Sn]]<sub>3</sub> may be a [[heavy fermion material]].<ref name="Yoshida720" /> Investigations are underway regarding alloys of neptunium with uranium, [[americium]], [[plutonium]], [[zirconium]], and [[iron]], so as to recycle long-lived waste isotopes such as neptunium-237 into shorter-lived isotopes more useful as nuclear fuel.<ref name="Yoshida720" />

One neptunium-based [[superconductivity|superconductor]] alloy has been discovered with formula Np[[palladium|Pd]]<sub>5</sub>Al<sub>2</sub>. This occurrence in neptunium compounds is somewhat surprising because they often exhibit strong magnetism, which usually destroys superconductivity. The alloy has a tetragonal structure with a superconductivity transition temperature of −268.3&nbsp;°C (4.9&nbsp;K).<ref name="lanl" /><ref>{{cite journal |author=T. D. Matsuda |author2= Y. Hagal |author3= D. Aoki |author4= H. Sakai |author5= Y. Homma |author6= N. Tateiwa|author7=E. Yamamoto |author8=Y. Onuki |date=2009 |title=Transport properties of neptunium superconductor NpPd<sub>5</sub>Al<sub>2</sub> |journal=Journal of Physics: Conference Series |volume=150 |issue=4 | pages=042119 |doi=10.1088/1742-6596/150/4/042119|bibcode = 2009JPhCS.150d2119M |doi-access=free }}</ref>

===Chemical===
Neptunium has five ionic [[oxidation state]]s ranging from +3 to +7 when forming chemical compounds, which can be simultaneously observed in solutions. It is the heaviest actinide that can lose all its valence electrons in a stable compound. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of [[coordination compound]]s.<ref name="Himiya neptuniya">{{cite book|title = Analytical chemistry of neptunium|editor=V. A. Mikhailov|place =Moscow |publisher = [[Nauka (publisher)|Nauka]]|date = 1971}}</ref>

===Atomic===
A neptunium atom has 93 electrons, arranged in the [[electron configuration|configuration]] <nowiki>[</nowiki>[[Radon|Rn]]<nowiki>]</nowiki>&nbsp;5f<sup>4</sup>&nbsp;6d<sup>1</sup>&nbsp;7s<sup>2</sup>. This differs from the configuration expected by the [[Aufbau principle]] in that one electron is in the 6d [[Electron shell#Subshells|subshell]] instead of being as expected in the 5f subshell. This is because of the similarity of the electron energies of the 5f, 6d, and 7s subshells. In forming compounds and ions, all the valence electrons may be lost, leaving behind an inert core of inner electrons with the electron configuration of the [[noble gas]] radon;<ref>{{cite book|author = Golub, A. M. |title = Общая и неорганическая химия (General and Inorganic Chemistry)|date = 1971|volume = 2|pages=222–7}}</ref> more commonly, only some of the valence electrons will be lost. The electron configuration for the tripositive ion Np<sup>3+</sup> is [Rn]&nbsp;5f<sup>4</sup>, with the outermost 7s and 6d electrons lost first: this is exactly analogous to neptunium's lanthanide homolog promethium, and conforms to the trend set by the other actinides with their [Rn]&nbsp;5f<sup>''n''</sup> electron configurations in the tripositive state. The first [[ionization potential]] of neptunium was measured to be at most {{val|6.19|0.12|u=[[electronvolt|eV]]}} in 1974, based on the assumption that the 7s electrons would ionize before 5f and 6d;<ref name="NIST">{{cite journal |first1=W. C. |last1=Martin |first2=Lucy |last2=Hagan |first3=Joseph |last3=Reader |first4=Jack |last4=Sugan |date=1974 |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |url=https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |journal=J. Phys. Chem. Ref. Data |volume=3 |issue=3 |pages=771–9 |access-date=2013-10-19 |doi=10.1063/1.3253147 |bibcode=1974JPCRD...3..771M |archive-date=2014-02-11 |archive-url=https://web.archive.org/web/20140211144635/https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |url-status=dead }}</ref> more recent measurements have refined this to 6.2657&nbsp;eV.<ref>David R. Lide (ed), ''CRC Handbook of Chemistry and Physics, 84th Edition''. CRC Press. Boca Raton, Florida, 2003; Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions.</ref>

===Isotopes===
{{Main|Isotopes of neptunium}}
[[Image:Decay Chain(4n+1, Neptunium Series).svg|thumb|The 4''n'' + 1 [[decay chain]] of neptunium-237, commonly called the "neptunium series"]]
Twenty-four neptunium [[radioisotope]]s have been characterized, with the most stable being <sup>237</sup>Np with a [[half-life]] of 2.14 million years, <sup>236</sup>Np with a half-life of 154,000 years, and <sup>235</sup>Np with a half-life of 396.1 days. All of the remaining [[radioactive]] isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has at least four [[meta state]]s, with the most stable being <sup>236m</sup>Np with a half-life of 22.5 hours.<ref name="unc">{{cite web |url=http://www.nucleonica.net/unc.aspx |title=Universal Nuclide Chart |author=Nucleonica |date=2007–2013 |website=Nucleonica: Web Driven Nuclear Science |access-date=2013-10-15}} {{registration required}}.</ref>

The isotopes of neptunium range in [[atomic weight]] from 219.032 [[atomic mass unit|u]] (<sup>219</sup>Np) to 244.068 u (<sup>244</sup>Np), though <sup>221</sup>Np has not yet been reported.{{NUBASE2020|ref}} Most of the isotopes that are lighter than the most stable one, <sup>237</sup>Np, [[radioactive decay|decay]] primarily by [[electron capture]] although a sizable number, most notably <sup>229</sup>Np and <sup>230</sup>Np, also exhibit various levels of decay via [[alpha emission]] to become [[protactinium]]. <sup>237</sup>Np itself, being the [[beta-decay stable isobars|beta-stable isobar]] of mass number 237, decays almost exclusively by alpha emission into <sup>233</sup>[[Isotopes of protactinium|Pa]], with very rare (occurring only about once in trillions of decays) [[spontaneous fission]] and [[cluster decay]] (emission of <sup>30</sup>Mg to form <sup>207</sup>Tl). All of the known isotopes except one that are heavier than this decay exclusively via [[beta emission]].<ref name="unc" /><ref name="Yoshida702" /> The lone exception, <sup>240m</sup>Np, exhibits a rare (>0.12%) decay by [[isomeric transition]] in addition to beta emission.<ref name="unc" /> <sup>237</sup>Np eventually decays to form [[bismuth]]-209 and [[thallium]]-205, unlike most other common heavy nuclei which decay into [[isotopes of lead]]. This [[decay chain]] is known as the [[neptunium series]].<ref name="lanl">{{cite web| url=http://periodic.lanl.gov/93.shtml| title=Periodic Table Of Elements: LANL - Neptunium| publisher=Los Alamos National Laboratory| access-date=2013-10-13}}</ref><ref>{{cite book|author=C. M. Lederer|author2=J. M. Hollander|author3=I. Perlman|date=1968|title=Table of Isotopes|edition=6th|location=New York|publisher=[[John Wiley & Sons]]}}</ref> This decay chain had long been extinct on Earth due to the short half-lives of all of its isotopes above bismuth-209, but is now being resurrected thanks to artificial production of neptunium on the tonne scale.<ref>{{cite book|last1=Koch|first1=Lothar|title=Transuranium Elements, in Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a27_167|chapter=Transuranium Elements|isbn=978-3527306732}}</ref>

[[Image:Np sphere.jpg|thumb|This nickel-clad neptunium sphere was used to experimentally determine the critical mass of Np at Los Alamos National Lab.]]
The isotopes neptunium-235, -236, and -237 are predicted to be [[fissile]];<ref name="critical" /> only neptunium-237's fissionability has been experimentally shown, with the [[critical mass]] being about 60&nbsp;kg, only about 10&nbsp;kg more than that of the commonly used [[uranium-235]].<ref name="Weiss">{{cite journal |last=Weiss |first=Peter |date=2 July 2009 |title=Neptunium nukes?: Little-studied metal goes critical |journal=Science News |volume=162 |issue=17 |pages=259 |doi=10.2307/4014034 |jstor=4014034 }}</ref> Calculated values of the critical masses of neptunium-235, -236, and -237 respectively are 66.2&nbsp;kg, 6.79&nbsp;kg, and 63.6&nbsp;kg: the neptunium-236 value is even lower than that of [[plutonium-239]]. In particular, <sup>236</sup>Np also has a low neutron [[cross section (physics)|cross section]].<ref name="critical" /> Despite this, a neptunium [[atomic bomb]] has never been built:<ref name="Weiss" /> uranium and plutonium have lower critical masses than <sup>235</sup>Np and <sup>237</sup>Np, and <sup>236</sup>Np is difficult to purify as it is not found in quantity in [[spent nuclear fuel]]<ref name="Yoshida702" /> and is nearly impossible to separate in any significant quantities from <sup>237</sup>Np.<ref name="Jukka">{{cite book |author=Jukka Lehto |author2=Xiaolin Hou |date=2011|chapter=15.15: Neptunium |title=Chemistry and Analysis of Radionuclides |page=231 |no-pp=yes |edition=1st |publisher=[[John Wiley & Sons]] |isbn=978-3527633029}}</ref>

===Occurrence===
The longest-lived isotope of neptunium, <sup>237</sup>Np, has a half-life of 2.14 million years, which is more than 2,000 times shorter than the [[age of the Earth]]. Therefore, any [[primordial nuclide|primordial]] neptunium would have decayed in the distant past. After only about 80 million years, the concentration of even the longest-lived isotope, <sup>237</sup>Np, would have been reduced to less than one-trillionth (10<sup>−12</sup>) of its original amount.<ref name="Yoshida704">Yoshida et al., pp. 703–4.</ref> Thus neptunium is present in nature only in negligible amounts produced as intermediate decay products of other isotopes.<ref name="Himiya neptuniya" />

[[Trace radioisotope|Trace]] amounts of the neptunium isotopes neptunium-237 and -239 are found naturally as [[decay product]]s from [[Nuclear transmutation|transmutation]] reactions in [[uranium ore]]s.<ref name="CRC" /><ref name="emsley345347">Emsley, pp. 345–347.</ref> <sup>239</sup>Np and <sup>237</sup>Np are the most common of these isotopes; they are directly formed from [[neutron capture]] by uranium-238 atoms. These neutrons come from the [[spontaneous fission]] of uranium-238, naturally neutron-induced fission of uranium-235, [[cosmic ray spallation]] of nuclei, and light elements absorbing [[alpha particle]]s and emitting a neutron.<ref name="Yoshida704" /> The half-life of <sup>239</sup>Np is very short, although the detection of its much longer-lived [[daughter product|daughter]] <sup>239</sup>Pu in nature in 1951 definitively established its natural occurrence.<ref name="Yoshida704" /> In 1952, <sup>237</sup>Np was identified and isolated from concentrates of uranium ore from the [[Belgian Congo]]: in these minerals, the ratio of neptunium-237 to uranium is less than or equal to about 10<sup>−12</sup> to 1.<ref name="Yoshida704" /><ref name="thompson1to4">
{{cite journal |last=Thompson |first=Roy C. |date=1982 |title=Neptunium: The Neglected Actinide: A Review of the Biological and Environmental Literature |journal=Radiation Research |volume=90 |issue=1 |pages=1–32 |doi=10.2307/3575792 |pmid=7038752 |jstor=3575792 |bibcode=1982RadR...90....1T }}</ref><ref name="NUBASE">{{NUBASE 2003}}</ref> Additionally, <sup>240</sup>Np must also occur as an intermediate decay product of [[plutonium-244|<sup>244</sup>Pu]], which has been detected in meteorite dust in marine sediments on Earth.<ref name="WallnerFaestermann2015">{{cite journal|last1=Wallner|first1=A.|last2=Faestermann|first2=T.|last3=Feige|first3=J.|last4=Feldstein|first4=C.|last5=Knie|first5=K.|last6=Korschinek|first6=G.|last7=Kutschera|first7=W.|last8=Ofan|first8=A.|last9=Paul|first9=M.|last10=Quinto|first10=F.|last11=Rugel|first11=G.|last12=Steier|first12=P.|title=Abundance of live <sup>244</sup>Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis|journal=Nature Communications|volume=6|year=2015|pages=5956|issn=2041-1723|doi=10.1038/ncomms6956|pmid=25601158 |pmc=4309418 |arxiv=1509.08054|bibcode=2015NatCo...6.5956W}}</ref>

Most neptunium (and plutonium) now encountered in the environment is due to atmospheric nuclear explosions that took place between the detonation of the [[Trinity test|first atomic bomb]] in 1945 and the ratification of the [[Partial Nuclear Test Ban Treaty]] in 1963. The total amount of neptunium released by these explosions and the few atmospheric tests that have been carried out since 1963 is estimated to be around 2500&nbsp;kg. The overwhelming majority of this is composed of the long-lived isotopes <sup>236</sup>Np and <sup>237</sup>Np since even the moderately long-lived <sup>235</sup>Np (half-life 396&nbsp;days) would have decayed to less than one-billionth (10<sup>−9</sup>) its original concentration over the intervening decades. An additional very small amount of neptunium, produced by neutron irradiation of natural uranium in nuclear reactor cooling water, is released when the water is discharged into rivers or lakes.<ref name="Yoshida704" /><ref name="thompson1to4" /><ref>{{cite book |last=Foster |first=R. F. |title=Environmental behavior of chromium and neptunium ''in'' Radioecology |date=1963 |publisher=Reinhold |location=New York |pages=569–576}}</ref> The concentration of <sup>237</sup>Np in seawater is approximately 6.5&nbsp;×&nbsp;10<sup>−5</sup>&nbsp;[[becquerel (unit)|millibecquerels]] per [[liter]]: this concentration is between 0.1% and 1% that of plutonium.<ref name="Yoshida704" />

Once released in the surface environment, in contact with atmospheric [[oxygen]], neptunium generally [[oxidation|oxidizes]] fairly quickly, usually to the +4 or +5 state. Regardless of its [[oxidation state]], the element exhibits much greater mobility than the other actinides, largely due to its ability to readily form aqueous solutions with various other elements. In one study comparing the diffusion rates of neptunium(V), plutonium(IV), and americium(III) in sandstone and limestone, neptunium penetrated more than ten times as well as the other elements. Np(V) will also react efficiently in pH levels greater than 5.5 if there are no [[carbonate]]s present and in these conditions it has also been observed to readily bond with [[quartz]]. It has also been observed to bond well with [[goethite]], [[ferric oxide]] colloids, and several clays including [[kaolinite]] and [[smectite]]. Np(V) does not bond as readily to soil particles in mildly acidic conditions as its fellow actinides americium and curium by nearly an order of magnitude. This behavior enables it to migrate rapidly through the soil while in solution without becoming fixed in place, contributing further to its mobility.<ref name="thompson1to4" /><ref name="atwood4">Atwood, section 4.</ref> Np(V) is also readily absorbed by [[concrete]], which because of the element's radioactivity is a consideration that must be addressed when building [[nuclear waste]] storage facilities. When absorbed in concrete, it is [[Redox|reduced]] to Np(IV) in a relatively short period of time. Np(V) is also reduced by [[humic acid]]s if they are present on the surface of goethite, [[hematite]], and [[magnetite]]. Np(IV) is less mobile and efficiently [[Sorption|adsorbed]] by [[tuff]], [[granodiorite]], and [[bentonite]]; although uptake by the latter is most pronounced in mildly acidic conditions. It also exhibits a strong tendency to bind to [[colloid|colloidal particulates]], an effect that is enhanced when in surface [[soil]] with high [[clay]] content. The behavior provides an additional aid in the element's observed high mobility.<ref name="thompson1to4" /><ref name="atwood4" /><ref name="atwood1">Atwood, section 1.</ref><ref>{{cite web| url=http://hpschapters.org/northcarolina/NSDS/neptunium.pdf| title=Human Health Fact Sheet - Neptunium| publisher=Health Physics Society| date=2001| access-date=2013-10-15}}</ref>

==History==

===Background and early claims===
[[File:Mendelejevs periodiska system 1871.png|thumb|right|alt=a table with a typical cell containing a two-letter symbol and a number|[[Dmitri Mendeleev]]'s table of 1871, with an empty space at the position after uranium]]
When the first [[periodic table]] of the elements was published by [[Dmitri Mendeleev]] in the early 1870s, it showed a "&nbsp;—&nbsp;" in place after uranium similar to several other places for then-undiscovered elements. Other subsequent tables of known elements, including a 1913 publication of the known radioactive isotopes by [[Kasimir Fajans]], also show an empty place after uranium, element 92.<ref>{{cite journal | last1 = Fajans | first1 = Kasimir | title = Die radioaktiven Umwandlungen und das periodische System der Elemente (Radioactive Transformations and the Periodic Table of the Elements) | journal = Berichte der Deutschen Chemischen Gesellschaft | volume = 46 | pages = 422–439 | date = 1913 | doi = 10.1002/cber.19130460162| url = https://zenodo.org/record/1426497 }}</ref>

Up to and after the discovery of the final component of the atomic nucleus, the [[neutron]] in 1932, most scientists did not seriously consider the possibility of elements heavier than uranium. While nuclear theory at the time did not explicitly prohibit their existence, there was little evidence to suggest that they did. However, the discovery of [[induced radioactivity]] by [[Irène Joliot-Curie|Irène]] and [[Frédéric Joliot-Curie]] in late 1933 opened up an entirely new method of researching the elements and inspired a small group of Italian scientists led by [[Enrico Fermi]] to begin a series of experiments involving neutron bombardment. Although the Joliot-Curies' experiment involved bombarding a sample of <sup>27</sup>[[Isotopes of aluminium|Al]] with [[alpha particle]]s to produce the radioactive <sup>30</sup>[[Isotopes of phosphorus|P]], Fermi realized that using neutrons, which have no electrical charge, would most likely produce even better results than the positively charged alpha particles. Accordingly, in March 1934 he began systematically subjecting all of the then-known elements to neutron bombardment to determine whether others could also be induced to radioactivity.<ref>Rhodes, pp. 201–202.</ref><ref>Rhodes, pp. 209–210.</ref>

After several months of work, Fermi's group had tentatively determined that lighter elements would disperse the energy of the captured neutron by emitting a [[proton]] or [[alpha particle]] and heavier elements would generally accomplish the same by emitting a [[gamma ray]]. This latter behavior would later result in the [[beta decay]] of a neutron into a proton, thus moving the resulting isotope one place up the periodic table. When Fermi's team bombarded uranium, they observed this behavior as well, which strongly suggested that the resulting isotope had an [[atomic number]] of 93. Fermi was initially reluctant to publicize such a claim, but after his team observed several unknown half-lives in the uranium bombardment products that did not match those of any known isotope, he published a paper entitled ''Possible Production of Elements of Atomic Number Higher than 92'' in June 1934. For element 93, he proposed the name ''[[ausenium]]'' (atomic symbol Ao)  after the Greek name ''Ausonia'' for Italy.<ref name="Fermi">{{cite journal | doi =10.1038/133898a0 | title =Possible Production of Elements of Atomic Number Higher than 92 | date =1934 | author =Fermi, E. | journal =Nature | volume =133 | pages =898–899 | bibcode=1934Natur.133..898F | issue =3372| doi-access =free }}</ref><ref>Enrico Fermi, [https://www.nobelprize.org/uploads/2018/06/fermi-lecture.pdf Artificial radioactivity produced by neutron bombardment], Nobel Lecture, December 12, 1938.</ref>

Several theoretical objections to the claims of Fermi's paper were quickly raised; in particular, the exact process that took place when an atom [[Neutron capture|captured a neutron]] was not well understood at the time. This and Fermi's accidental discovery three months later that nuclear reactions could be induced by slow neutrons cast further doubt in the minds of many scientists, notably [[Aristid von Grosse]] and [[Ida Noddack]], that the experiment was creating element 93. While von Grosse's claim that Fermi was actually producing [[protactinium]] (element 91) was quickly tested and disproved, Noddack's proposal that the uranium had been shattered into two or more much smaller fragments was simply ignored by most because existing nuclear theory did not include a way for this to be possible. Fermi and his team maintained that they were in fact synthesizing a new element, but the issue remained unresolved for several years.<ref>Hoffman, pp. 120–123.</ref><ref>{{cite journal|author=Ida Noddack|author-link=Ida Noddack|date=1934|pages=653–655|title=Über das Element 93|volume=47|journal=Zeitschrift für Angewandte Chemie|url=http://www.chemteam.info/Chem-History/Noddack-1934.html|doi=10.1002/ange.19340473707|issue=37|bibcode=1934AngCh..47..653N}}</ref><ref>Rhodes, pp. 210–220.</ref>

Although the many different and unknown radioactive half-lives in the experiment's results showed that several nuclear reactions were occurring, Fermi's group could not prove that element 93 was being produced unless they could isolate it chemically. They and many other scientists attempted to accomplish this, including [[Otto Hahn]] and [[Lise Meitner]] who were among the best radiochemists in the world at the time and supporters of Fermi's claim, but they all failed. Much later, it was determined that the main reason for this failure was because the predictions of element 93's chemical properties were based on a periodic table which lacked the [[actinide series]]. This arrangement placed protactinium below tantalum, uranium below tungsten, and further suggested that element 93, at that point referred to as eka-rhenium, should be similar to the [[group 7 element]]s, including manganese and rhenium. Thorium, protactinium, and uranium, with their dominant oxidation states of +4, +5, and +6 respectively, fooled scientists into thinking they belonged below hafnium, tantalum, and tungsten, rather than below the lanthanide series, which was at the time viewed as a fluke, and whose members all have dominant +3 states; neptunium, on the other hand, has a much rarer, more unstable +7 state, with +4 and +5 being the most stable. Upon finding that [[plutonium]] and the other transuranic elements also have dominant +3 and +4 states, along with the discovery of the [[f-block]], the actinide series was firmly established.<ref>Rhodes, pp. 221–222.</ref><ref>Rhodes, p. 349.</ref>

While the question of whether Fermi's experiment had produced element 93 was stalemated, two additional claims of the discovery of the element appeared, although unlike Fermi, they both claimed to have observed it in nature. The first of these claims was by Czech engineer [[Odolen Koblic]] in 1934 when he extracted a small amount of material from the wash water of heated [[pitchblende]]. He proposed the name [[bohemium]] for the element, but after being analyzed it turned out that the sample was a mixture of [[tungsten]] and [[vanadium]].<ref name="Koblic">{{cite journal | first = Odolen | last = Koblic | doi =10.1038/134055b0 | title =A New Radioactive Element beyond Uranium | date =1934 | journal =Nature | volume =134 | pages =55 | issue=3376|bibcode = 1934Natur.134R..55. | doi-access =free }}</ref><ref>Hoffman, p. 118.</ref><ref>{{cite journal | doi =10.1126/science.80.2086.588-a | title =Bohemium - An Obituary | date =1934 | author =Speter, M. | journal =Science | volume =80 | pages =588–9 | pmid =17798409 | issue =2086|bibcode = 1934Sci....80..588S }}</ref> The other claim, in 1938 by Romanian physicist [[Horia Hulubei]] and French chemist [[Yvette Cauchois]], claimed to have discovered the new element via [[spectroscopy]] in minerals. They named their element [[sequanium]], but the claim was discounted because the prevailing theory at the time was that if it existed at all, element 93 would not exist naturally. However, as neptunium does in fact occur in nature in trace amounts, as demonstrated when it was found in uranium ore in 1952, it is possible that Hulubei and Cauchois did in fact observe neptunium.<ref name="emsley345347" /><ref name="fontani">{{cite conference| first = Marco| last = Fontani| title = The Twilight of the Naturally-Occurring Elements: Moldavium (Ml), Sequanium (Sq) and Dor (Do)| book-title = International Conference on the History of Chemistry| pages = 1–8| date = 2005| location = Lisbon| url = http://5ichc-portugal.ulusofona.pt/uploads/PaperLong-MarcoFontani.doc| archive-url = https://web.archive.org/web/20060224090117/http://5ichc-portugal.ulusofona.pt/uploads/PaperLong-MarcoFontani.doc| archive-date=2006-02-24| access-date = 2013-10-13 }}</ref><ref>{{cite journal|title = Nouvelles recherches sur l'élément 93 naturel|first = H.|last = Hulubei|author2=Cauchois, Y.|journal = Comptes Rendus|date = 1939|volume = 209|pages = 476–479|url = http://gallica.bnf.fr/ark:/12148/bpt6k3161s.image.f478.langFR}}</ref><ref name=Peppard>{{cite journal | last1 =Peppard | first1 = D. F. | title =Occurrence of the (4n + 1) Series in Nature | last2 =Mason | first2 = G. W. | last3 =Gray | first3 = P. R. | last4 =Mech | first4 = J. F. | journal =Journal of the American Chemical Society | volume =74 | pages =6081–6084 | date =1952 | doi =10.1021/ja01143a074 | issue =23| bibcode = 1952JAChS..74.6081P | url = https://digital.library.unt.edu/ark:/67531/metadc172698/ }}</ref>

Although by 1938 some scientists, including [[Niels Bohr]], were still reluctant to accept that Fermi had actually produced a new element, he was nevertheless awarded the [[Nobel Prize in Physics]] in November 1938 "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". A month later, the almost totally unexpected discovery of [[nuclear fission]] by Hahn, Meitner, and [[Otto Frisch]] put an end to the possibility that Fermi had discovered element 93 because most of the unknown half-lives that had been observed by Fermi's team were rapidly identified as those of [[fission products]].<ref>Rhodes, pp. 264–267.</ref><ref>Rhodes, p. 346.</ref><ref>{{cite web |title=The Nobel Prize in Physics 1938 |publisher=Nobel Foundation |url=http://nobelprize.org/nobel_prizes/physics/laureates/1938/index.html |access-date=2013-10-13}}</ref><ref>{{cite journal|last1=Meitner|first1=Lise|last2=Frisch|first2=O. R.|doi=10.1038/143239a0|title=Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction |date=1939|pages=239–240|volume=143|journal=Nature|url=http://www.nature.com/physics/looking-back/meitner/index.html|bibcode=1939Natur.143..239M|issue=3615|s2cid=4113262}}</ref><ref>{{cite journal|url=http://www.crownedanarchist.com/emc2/discovery_of_fission.doc|archive-url=https://web.archive.org/web/20101224154146/http://www.crownedanarchist.com/emc2/discovery_of_fission.doc|url-status=dead|archive-date=2010-12-24|title=Discovery of fission|author=Otto Hahn|journal=Scientific American|date=1958}}</ref>

Perhaps the closest of all attempts to produce the missing element 93 was that conducted by the Japanese physicist [[Yoshio Nishina]] working with chemist [[Kenjiro Kimura]] in 1940, just before the outbreak of the [[Pacific War]] in 1941: they bombarded [[uranium-238|<sup>238</sup>U]] with fast neutrons. However, while slow neutrons tend to induce neutron capture through a (n, γ) reaction, fast neutrons tend to induce a "knock-out" (n, 2n) reaction, where one neutron is added and two more are removed, resulting in the net loss of a neutron. Nishina and Kimura, having tested this technique on <sup>232</sup>[[thorium|Th]] and successfully produced the known <sup>231</sup>Th and its long-lived beta decay daughter <sup>231</sup>[[protactinium|Pa]] (both occurring in the natural decay chain of [[uranium-235|<sup>235</sup>U]]), therefore correctly assigned the new 6.75-day half-life activity they observed to the new isotope <sup>237</sup>U. They confirmed that this isotope was also a beta emitter and must hence decay to the unknown nuclide <sup>237</sup>93. They attempted to isolate this nuclide by carrying it with its supposed lighter congener rhenium, but no beta or alpha decay was observed from the rhenium-containing fraction: Nishina and Kimura thus correctly speculated that the half-life of <sup>237</sup>93, like that of <sup>231</sup>Pa, was very long and hence its activity would be so weak as to be unmeasurable by their equipment, thus concluding the last and closest unsuccessful search for transuranic elements.<ref name="Ikeda">{{cite journal |last1=Ikeda |first1=Nagao |date=25 July 2011 |title=The discoveries of uranium 237 and symmetric fission — From the archival papers of Nishina and Kimura |journal=Proceedings of the Japan Academy, Series B: Physical and Biological Sciences |volume=87 |issue=7 |pages=371–6 |doi=10.2183/pjab.87.371 |bibcode=2011PJAB...87..371I |pmc=3171289 |pmid=21785255}}</ref>

===Discovery===
[[File:Berkeley 60-inch cyclotron.jpg|thumb|The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939|alt=Black-and-white picture of heavy machinery with two operators sitting aside.]]
As research on nuclear fission progressed in early 1939, [[Edwin McMillan]] at the [[Berkeley Radiation Laboratory]] of the [[University of California, Berkeley]] decided to run an experiment bombarding uranium using the powerful 60-inch (1.52 m) [[cyclotron]] that had recently been built at the university. The purpose was to separate the various fission products produced by the bombardment by exploiting the enormous force that the fragments gain from their mutual electrical repulsion after fissioning. Although he did not discover anything of note from this, McMillan did observe two new beta decay half-lives in the uranium trioxide target itself, which meant that whatever was producing the radioactivity had not violently repelled each other like normal fission products. He quickly realized that one of the half-lives closely matched the known 23-minute decay period of uranium-239, but the other half-life of 2.3 days was unknown. McMillan took the results of his experiment to chemist and fellow Berkeley professor [[Emilio Segrè]] to attempt to isolate the source of the radioactivity. Both scientists began their work using the prevailing theory that element 93 would have similar chemistry to rhenium, but Segrè rapidly determined that McMillan's sample was not at all similar to rhenium. Instead, when he reacted it with [[hydrogen fluoride]] (HF) with a strong [[oxidizing agent]] present, it behaved much like members of the [[rare earths]]. Since these elements comprise a large percentage of fission products, Segrè and McMillan decided that the half-life must have been simply another fission product, titling the paper "An Unsuccessful Search for Transuranium Elements".<ref>{{cite journal| last1=Segrè| first1=Emilio| title=An Unsuccessful Search for Transuranium Elements| date=1939| pages=1104–5| journal=Physical Review|volume=55| issue=11|bibcode = 1939PhRv...55.1104S |doi = 10.1103/PhysRev.55.1104 }}</ref><ref>Rhodes, pp. 346–350.</ref><ref>Yoshida et al., pp. 699–700.</ref>
 
However, as more information about fission became available, the possibility that the fragments of nuclear fission could still have been present in the target became more remote. McMillan and several scientists, including [[Philip H. Abelson]], attempted again to determine what was producing the unknown half-life. In early 1940, McMillan realized that his 1939 experiment with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor. In a new experiment, McMillan tried subjecting the unknown substance to HF in the presence of a [[reducing agent]], something he had not done before. This reaction resulted in the sample [[Precipitation (chemistry)|precipitating]] with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare-earth metal. 
Shortly after this, Abelson, who had received his [[Doctor of Science|graduate degree]] from the university, visited Berkeley for a short vacation and McMillan asked the more able chemist to assist with the separation of the experiment's results. Abelson very quickly observed that whatever was producing the 2.3-day half-life did not have chemistry like any known element and was actually more similar to uranium than a rare-earth metal. This discovery finally allowed the source to be isolated and later, in 1945, led to the classification of the [[actinide series]]. As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from <sup>239</sup>U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction:<ref name="EL93" />
:<chem>{^{238}_{92}U} + {^{1}_{0}n} -> {^{239}_{92}U} ->[\beta^-][23\ \ce{min}] {^{239}_{93}Np} ->[\beta^-][2.355\ \ce{days}] {^{239}_{94}Pu}</chem> <small>''(The times are [[half-life|half-lives]].)''</small>

This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered. McMillan and Abelson published their results in a paper entitled ''Radioactive Element 93'' in the ''[[Physical Review]]'' on May 27, 1940.<ref name="EL93">{{cite journal| doi =10.1103/PhysRev.57.1185.2| title =Radioactive Element 93| date =1940| author =Mcmillan, Edwin| journal =Physical Review| volume =57| pages =1185–1186| last2 =Abelson| first2 =Philip| issue =12|bibcode = 1940PhRv...57.1185M | doi-access =free}}</ref> They did not propose a name for the element in the paper, but they soon decided on the name ''neptunium'' since [[Neptune]] is the next planet beyond [[Uranus]] in our solar system, which uranium is named after.<ref name="lanl" /><ref>{{cite book|title=Handbook on the Physics and Chemistry of Rare Earths|volume=18 – Lanthanides/Actinides: Chemistry|editor=K. A. Gschneidner, Jr.|editor2=L, Eyring|editor3=G. R. Choppin|editor4=G. H. Landet|date=1994 |publisher=Elsevier|chapter= Ch. 118. Origin of the actinide concept|author=Seaborg, G. T.|pages=4–6, 10–14}}</ref><ref>Rhodes, pp. 348–350.</ref><ref>Yoshida et al., p. 700.</ref><ref>The name ''neptunium'' was used by R.Hermann in 1877 for a chemical element which, in his opinion, could be separated from a mineral tantalite; actually, this was a misidentification. See {{cite journal| title =Chemical Notes : The New Metals Ilmenium and Neptunium| date =1877| url=https://www.nature.com/articles/015520a0| journal =Nature| volume =15| pages =520–521| doi =10.1038/015520a0}}</ref> McMillan and Abelson's success compared to Nishina and Kimura's near miss can be attributed to the favorable half-life of <sup>239</sup>Np for radiochemical analysis and quick decay of <sup>239</sup>U, in contrast to the slower decay of <sup>237</sup>U and extremely long half-life of <sup>237</sup>Np.<ref name="Ikeda" />

===Subsequent developments=== 
It was also realized that the beta decay of <sup>239</sup>Np must produce an isotope of element 94 (now called [[plutonium]]), but the quantities involved in McMillan and Abelson's original experiment were too small to isolate and identify plutonium along with neptunium.<ref>{{cite book|first1 = David L.|last1 = Clark|first2 = Siegfried S.|last2 = Hecker|first3 = Gordon D.|last3 = Jarvinen|first4 = Mary P.|last4 = Neu|contribution = Neptunium|title = The Chemistry of the Actinide and Transactinide Elements|editor1-first = Lester R.|editor1-last = Morss|editor2-first = Norman M.|editor2-last = Edelstein|editor3-first = Jean|editor3-last = Fuger|edition = 3rd|date = 2006|volume = 3|publisher = Springer|location = Dordrecht, the Netherlands|page = 814|url = http://radchem.nevada.edu/classes/rdch710/files/plutonium.pdf|doi = 10.1007/1-4020-3598-5_7|isbn = 978-1-4020-3555-5|access-date = 2014-06-29|archive-date = 2010-07-17|archive-url = https://web.archive.org/web/20100717155138/http://radchem.nevada.edu/classes/rdch710/files/Plutonium.pdf|url-status = dead}}</ref> The discovery of plutonium had to wait until the end of 1940, when [[Glenn T. Seaborg]] and his team identified the isotope [[plutonium-238]].<ref>{{cite journal|title=The plutonium story|author=Glenn T. Seaborg |publisher=Lawrence Berkeley Laboratory, University of California |id=LBL-13492, DE82 004551 |url=http://www.osti.gov/bridge/purl.cover.jsp?purl=/5808140-l5UMe1/|date=September 1981 }}</ref>

In 1942, Hahn and [[Fritz Strassmann]], and independently [[Kurt Starke]], reported the confirmation of element 93 in Berlin.<ref name="Germans">{{cite journal |last1=Winterberg |first1=Friedwardt |last2=Herrmann |first2=Günter |last3=Fodor |first3=Igor |last4=Wolfenstein |first4=Lincoln |last5=Singer |first5=Mark E. |date=1996 |title=More on How Nazi Germany Failed to Develop the Atomic Bomb |journal=Physics Today |volume=49 |issue=1 |pages=11–15, 83 |doi=10.1063/1.2807455|bibcode=1996PhT....49a..11W }}</ref> Hahn's group did not pursue element 94, likely because they were discouraged by McMillan and Abelson's lack of success in isolating it. Since they had access to the stronger cyclotron at Paris at this point, Hahn's group would likely have been able to detect element 94 had they tried, albeit in tiny quantities (a few [[becquerel (unit)|becquerels]]).<ref name="Germans" />

Neptunium's unique radioactive characteristics allowed it to be traced as it moved through various compounds in chemical reactions, at first this was the only method available to prove that its chemistry was different from other elements. As the first isotope of neptunium to be discovered has such a short half-life, McMillan and Abelson were unable to prepare a sample that was large enough to perform chemical analysis of the new element using the technology that was then available. However, after the discovery of the long-lived <sup>237</sup>Np isotope in 1942 by [[Glenn Seaborg]] and [[Arthur Wahl]], forming weighable amounts of neptunium became a realistic endeavor.<ref name="lanl" /><ref name="migration" /> Its half-life was initially determined to be about 3&nbsp;million years (later revised to 2.144&nbsp;million years), confirming the predictions of Nishina and Kimura of a very long half-life.<ref name="Ikeda" />

Early research into the element was somewhat limited because most of the nuclear physicists and chemists in the United States at the time were focused on the massive effort to research the properties of plutonium as part of the [[Manhattan Project]]. Research into the element did continue as a minor part of the project and the first bulk sample of neptunium (as [[neptunium dioxide]]) was isolated in 1944.<ref name="lanl" /><ref name="migration">{{cite book | url = https://books.google.com/books?id=1lArAAAAYAAJ | title = Radiochemistry of neptunium | author1 = Burney, G. A | author2 = Harbour, R. M | author3 = Subcommittee On Radiochemistry, National Research Council (U.S.) | author4 = Technical Information Center, U.S. Atomic Energy Commission | date = 1974}}</ref><ref>{{cite book | url =https://books.google.com/books?id=UnQ_NQAACAAJ | title =The migration chemistry of neptunium | isbn =978-87-550-1535-7 | author1 =Nilsson, Karen | date =1989| publisher =Risø National Laboratory }}</ref>

Much of the research into the properties of neptunium since then has been focused on understanding how to confine it as a portion of nuclear waste. Because it has isotopes with very long half-lives, it is of particular concern in the context of designing confinement facilities that can last for thousands of years. It has found some limited uses as a radioactive tracer and a precursor for various nuclear reactions to produce useful plutonium isotopes. However, most of the neptunium that is produced as a reaction byproduct in nuclear power stations is considered to be a waste product.<ref name="lanl" /><ref name="migration" />

==Production==
[[File:Neptunium Purex process.png|thumb|right|upright=1.4|[[Flowchart]], showing the Purex process and the likely oxidation state of neptunium in the process solution<ref name="Yoshida710" />]]

===Synthesis===
The vast majority of the neptunium that currently exists on Earth was produced artificially in nuclear reactions. Neptunium-237 is the most commonly synthesized isotope due to it being the only one that both can be produced via [[neutron capture]] and also has a half-life long enough to allow weighable quantities to be easily isolated. It is by far the most common isotope to be utilized in chemical studies of the element.<ref name="Yoshida702">Yoshida et al., p. 700–2.</ref>
* When an [[Uranium-235|<sup>235</sup>U]] atom captures a neutron, it is converted to an excited state of [[Uranium-236|<sup>236</sup>U]]. About 85.5%<ref name=power-ratio>{{cite web |url=https://www.nuclear-power.com/nuclear-power/fission/capture-to-fission-ratio/ |title=Capture-to-fission Ratio |website=nuclear-power.com |access-date=June 26, 2024}}</ref><ref name=ratio-1962>{{cite journal |title=The ratio of neutron capture to fission for uranium-235 |first1=M. J. |last1=Cabell |first2=L. J. |last2=Slee |date=1962 |journal=Journal of Inorganic and Nuclear Chemistry |volume=24 |issue=12 |pages=1493–1500 |doi=10.1016/0022-1902(62)80002-5}}</ref> of the excited <sup>236</sup>U nuclei undergo fission, but the remainder decay to the ground state of <sup>236</sup>U by emitting [[gamma radiation]]. Further neutron capture forms <sup>237</sup>U which has a half-life of 7 days and quickly decays to <sup>237</sup>Np through [[beta decay]]. During beta decay, the excited <sup>237</sup>U emits an electron, while the atomic [[weak interaction]] converts a [[neutron]] to a [[proton]], thus creating <sup>237</sup>Np.<ref name="Yoshida702" />

::<math chem>\begin{cases}
\ce{^{235}_{92}U + ^{1}_{0}n -> ^{236m}_{92}U ->[][120 \ \ce{ns}] ^{236}_{92}U + \gamma}\\
\ce{^{236}_{92}U + ^{1}_{0}n -> ^{237}_{92}U ->[\beta^-][6.75 \ \ce{d}] ^{237}_{93}Np}
\end{cases}</math>

* <sup>237</sup>U is also produced via an ([[neutron|n]],2n) reaction with [[Uranium-238|<sup>238</sup>U]]. This only happens with very energetic neutrons.<ref name="Yoshida702" />
* <sup>237</sup>Np is the product of [[alpha decay]] of [[Americium-241|<sup>241</sup>Am]], which is produced through neutron irradiation of [[uranium-238]].<ref name="Yoshida702" />

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled [[spent nuclear fuel]] gives nearly pure <sup>237</sup>Np.<ref name="Yoshida702" /> The short-lived heavier isotopes <sup>238</sup>Np and <sup>239</sup>Np, useful as [[radioactive tracer]]s, are produced through neutron irradiation of <sup>237</sup>Np and <sup>238</sup>U respectively, while the longer-lived lighter isotopes <sup>235</sup>Np and <sup>236</sup>Np are produced through irradiation of <sup>235</sup>U with [[proton]]s and [[deuteron]]s in a [[cyclotron]].<ref name="Yoshida702" />

Artificial <sup>237</sup>Np metal is usually isolated through a reaction of <sup>237</sup>NpF<sub>3</sub> with liquid [[barium]] or [[lithium]] at around 1200&nbsp;°[[Celsius|C]] and is most often extracted from spent [[nuclear fuel rod]]s in kilogram amounts as a by-product in [[plutonium]] production.<ref name="emsley345347" />

:2 NpF<sub>3</sub> + 3 Ba → 2 Np + 3 BaF<sub>2</sub>

By weight, neptunium-237 discharges are about 5% as great as plutonium discharges and about 0.05% of spent nuclear fuel discharges.<ref>{{cite web| url = http://www.isis-online.org/publications/fmct/book/New%20chapter%205.pdf| title = Separated Neptunium 237 and Americium| access-date = 2009-06-06}}</ref> However, even this fraction still amounts to more than fifty tons per year globally.<ref name="rsc">{{Cite web | url=http://www.rsc.org/chemistryworld/podcast/interactive_periodic_table_transcripts/neptunium.asp. |title = Chemistry news, research and opinions}}</ref>

===Purification methods===
Recovering uranium and plutonium from spent nuclear fuel for reuse is one of the major processes of the [[nuclear fuel cycle]]. As it has a long half-life of just over 2&nbsp;million years, the [[alpha decay|alpha emitter]] <sup>237</sup>Np is one of the major isotopes of the [[minor actinide]]s separated from spent nuclear fuel.<ref name="Yoshida7045">Yodshida et al., pp. 704–5.</ref> Many separation methods have been used to separate out the neptunium, operating on small and large scales. The small-scale purification operations have the goals of preparing pure neptunium as a [[precursor (chemistry)|precursor]] of metallic neptunium and its compounds, and also to isolate and preconcentrate neptunium in samples for analysis.<ref name="Yoshida7045" />

Most methods that separate neptunium ions exploit the differing chemical behaviour of the differing oxidation states of neptunium (from +3 to +6 or sometimes even +7) in solution.<ref name="Yoshida7045" /> Among the methods that are or have been used are: solvent [[extraction (chemistry)|extraction]] (using various [[extractant]]s, usually [[denticity|multidentate]] β-diketone derivatives, [[organophosphorus compound]]s, and [[amine]] compounds), [[chromatography]] using various [[ion exchange|ion-exchange]] or [[chelation|chelating]] resins, [[coprecipitation]] (possible [[matrix (chemical analysis)|matrices]] include [[lanthanum(III) fluoride|LaF<sub>3</sub>]], [[bismuth phosphate|BiPO<sub>4</sub>]], [[barium sulfate|BaSO<sub>4</sub>]], [[iron(III) hydroxide|Fe(OH)<sub>3</sub>]], and [[manganese(IV) oxide|MnO<sub>2</sub>]]), [[electroplating|electrodeposition]], and [[biotechnology|biotechnological]] methods.<ref name="Yoshida70517">Yoshida et al., pp. 705–17.</ref> Currently, commercial reprocessing plants use the Purex process, involving the solvent extraction of uranium and plutonium with [[tributyl phosphate]].<ref name="Yoshida710">Yoshida et al., p. 710.</ref>

==Chemistry and compounds==
{{Main|Neptunium compounds}}

===Solution chemistry===
[[File:Np ox st.jpg|thumb|upright=1.6|right|Neptunium ions in solution]]
When it is in an aqueous solution, neptunium can exist in any of its five possible oxidation states (+3 to +7) and each of these show a characteristic color.<ref name="Dutkiewicz2017" /> The stability of each oxidation state is strongly dependent on various factors, such as the presence of [[oxidizing agent|oxidizing]] or [[reducing agent]]s, [[pH]] of the solution, presence of [[coordination complex]]-forming [[ligand]]s, and even the concentration of neptunium in the solution.<ref name="Yoshida753">Yoshida et al., pp. 752–4.</ref>

{| class="wikitable" style="float:right; clear:right; margin-left:1em; margin-top:0;"
|-
! Oxidation <br />state !! Representative compound
|-
| +2 || [K(2.2.2-crypt)][NpCp'<sub>3</sub>]
|-
| +3 || [[Neptunium(III) chloride]], NpCl<sub>3</sub>
|-
| +4 || [[Neptunium(IV) oxide]], NpO<sub>2</sub>
|-
| +5 || [[Neptunium(V) fluoride]], NpF<sub>5</sub>
|-
| +6 || [[Neptunium(VI) fluoride]], NpF<sub>6</sub>
|-
| +7 || [[Neptunium(VII) oxide-hydroxide]], NpO<sub>2</sub>(OH)<sub>3</sub> 
|}
In [[acid (chemistry)|acidic]] solutions, the neptunium(III) to neptunium(VII) ions exist as Np<sup>3+</sup>, Np<sup>4+</sup>, {{chem|NpO|2|+}}, {{chem|NpO|2|2+}}, and {{chem|NpO|3|+}}. In [[base (chemistry)|basic]] solutions, they exist as the oxides and hydroxides Np(OH)<sub>3</sub>, NpO<sub>2</sub>, NpO<sub>2</sub>OH, NpO<sub>2</sub>(OH)<sub>2</sub>, and {{chem|NpO|5|3-}}. Not as much work has been done to characterize neptunium in basic solutions.<ref name="Yoshida753" /> Np<sup>3+</sup> and Np<sup>4+</sup> can easily be reduced and oxidized to each other, as can {{chem|NpO|2|+}} and {{chem|NpO|2|2+}}.<ref name="Yoshida759">Yoshida et al., p. 759.</ref>

;Neptunium(III)
Np(III) or Np<sup>3+</sup> exists as hydrated complexes in acidic solutions, {{chem|Np|(H|2|O|)|''n''|3+}}.<ref name="lanl" /> It is a dark blue-purple and is analogous to its lighter [[congener (chemistry)|congener]], the pink [[rare-earth metal|rare-earth]] ion [[promethium|Pm<sup>3+</sup>]].<ref name="lanl" /><ref name="Greenwood">{{Greenwood&Earnshaw|page=1265}}</ref> In the presence of [[oxygen]], it is quickly oxidized to Np(IV) unless strong reducing agents are also present. Nevertheless, it is the second-least easily [[hydrolysis|hydrolyzed]] neptunium ion in water, forming the NpOH<sup>2+</sup> ion.<ref name="Yoshida768" /> Np<sup>3+</sup> is the predominant neptunium ion in solutions of pH&nbsp;4–5.<ref name="Yoshida768">Yoshida et al., p. 766–70.</ref>

;Neptunium(IV)
[[File:Np(IV) in 8 M HCl..jpg|thumb|Np(IV) in 8 M HCl.]]Np(IV) or Np<sup>4+</sup> is pale yellow-green in acidic solutions,<ref name="lanl" /> where it exists as hydrated complexes ({{chem|Np|(H|2|O|)|''n''|4+}}). It is quite unstable to hydrolysis in acidic aqueous solutions at pH&nbsp;1 and above, forming NpOH<sup>3+</sup>.<ref name="Yoshida768" /> In basic solutions, Np<sup>4+</sup> tends to hydrolyze to form the neutral neptunium(IV) hydroxide (Np(OH)<sub>4</sub>) and neptunium(IV) oxide (NpO<sub>2</sub>).<ref name="Yoshida768" />

;Neptunium(V)
Np(V) or {{chem|NpO|2|+}} is green-blue in aqueous solution,<ref name="lanl" /> in which it behaves as a strong [[Lewis acid]].<ref name="Yoshida753" /> It is a stable ion<ref name="Yoshida753" /> and is the most common form of neptunium in aqueous solutions. Unlike its neighboring homologues {{chem|UO|2|+}} and {{chem|PuO|2|+}}, {{chem|NpO|2|+}} does not spontaneously [[disproportionation|disproportionate]] except at very low pH and high concentration:<ref name="Yoshida759" />

:2 {{chem|NpO|2|+}} + 4 H<sup>+</sup> ⇌ Np<sup>4+</sup> + {{chem|NpO|2|2+}} + 2 H<sub>2</sub>O

It hydrolyzes in basic solutions to form NpO<sub>2</sub>OH and {{chem|NpO|2|(OH)|2|-}}.<ref name="Yoshida768" />

;Neptunium(VI)
Np(VI) or {{chem|NpO|2|2+}}, the neptunyl ion, shows a light pink or reddish color in an acidic solution and yellow-green otherwise.<ref name="lanl" /> It is a strong Lewis acid<ref name="Yoshida753" /> and is the main neptunium ion encountered in solutions of pH&nbsp;3–4.<ref name="Yoshida768" /> Though stable in acidic solutions, it is quite easily reduced to the Np(V) ion,<ref name="Yoshida753" /> and it is not as stable as the homologous hexavalent ions of its neighbours uranium and plutonium (the [[uranyl]] and [[plutonyl]] ions). It hydrolyzes in basic solutions to form the oxo and hydroxo ions NpO<sub>2</sub>OH<sup>+</sup>, {{chem|(NpO|2|)|2|(OH)|2|2+}}, and {{chem|(NpO|2|)|3|(OH)|5|+}}.<ref name="Yoshida768" />

;Neptunium(VII)
Np(VII) is dark green in a strongly [[Base (chemistry)|basic]] solution. Though its [[chemical formula]] in basic solution is frequently cited as {{chem|NpO|5|3-}}, this is a simplification and the real structure is probably closer to a hydroxo species like {{chem|[NpO|4|(OH)|2|]|3-}}.<ref name="lanl" /><ref name="Greenwood" /> Np(VII) was first prepared in basic solution in 1967.<ref name="Yoshida753" /> In strongly [[acid (chemistry)|acidic]] solution, Np(VII) is found as {{chem|NpO|3|+}}; water quickly reduces this to Np(VI).<ref name="Yoshida753" /> Its hydrolysis products are uncharacterized.<ref name="Yoshida768" />

===Hydroxides===
The oxides and hydroxides of neptunium are closely related to its ions. In general, Np hydroxides at various oxidation levels are less stable than the actinides before it on the periodic table such as [[thorium]] and uranium and more stable than those after it such as plutonium and americium. This phenomenon is because the stability of an ion increases as the ratio of atomic number to the radius of the ion increases. Thus actinides higher on the periodic table will more readily undergo [[hydrolysis]].<ref name="Yoshida753" /><ref name="Yoshida768" />

Neptunium(III) hydroxide is quite stable in acidic solutions and in environments that lack oxygen, but it will rapidly oxidize to the IV state in the presence of air. It is not soluble in water.<ref name="migration" /> Np(IV) hydroxides exist mainly as the electrically neutral Np(OH)<sub>4</sub> and its mild solubility in water is not affected at all by the pH of the solution. This suggests that the other Np(IV) hydroxide, {{chem|Np|(OH)|5|-}}, does not have a significant presence.<ref name="Yoshida768" /><ref>{{cite journal |author=Trygve E Eriksen |display-authors=4 |author2=Pierre Ndalamba |author3=Daqing Cui |author4=Jordi Bruno |author5=Marco Caceci |author6=Kastriot Spahiu |date=1993 |title=Solubility of the redox-sensitive radionuclides <sup>99</sup>Tc and <sup>237</sup>Np under reducing conditions in neutral to alkaline solutions. |journal=SKB Technical Report |volume=93-18 | pages=1–32 |url=http://www.skb.se/upload/publications/pdf/TR93-18webb.pdf}}</ref>

Because the Np(V) ion {{chem|NpO|2|+}} is very stable, it can only form a hydroxide in high acidity levels. When placed in a 0.1&nbsp;[[molar concentration|M]] [[sodium perchlorate]] solution, it does not react significantly for a period of months, although a higher molar concentration of 3.0&nbsp;M will result in it reacting to the solid hydroxide NpO<sub>2</sub>OH almost immediately. Np(VI) hydroxide is more reactive but it is still fairly stable in acidic solutions. It will form the compound NpO<sub>3</sub>· H<sub>2</sub>O in the presence of [[ozone]] under various [[carbon dioxide]] pressures. Np(VII) has not been well-studied and no neutral hydroxides have been reported. It probably exists mostly as {{chem|[NpO|4|(OH)|2|]|3-}}.<ref name="Yoshida768" /><ref>{{cite journal |author=Neck, V. |author2=Kim, J. I. |author3=Kanellakopulos, B. |date=1992 |title=Solubility and hydrolysis behaviour of neptunium(V) | journal=Radiochim. Acta |volume=56 | pages=25–30|doi=10.1524/ract.1992.56.1.25 |s2cid=99239460 }}</ref><ref>{{cite journal |author=Kato, Y. |author2=Kimura, T. |author3=Yoshida, Z. |author4=Nitani, N. |date=1996 |title=Solid-Liquid Phase-Equilibria Of Np(VI) And Of U(VI) Under Controlled CO<sub>2</sub> Partial Pressures |journal=Radiochim. Acta |volume=74 | pages=21–25|doi=10.1524/ract.1996.74.special-issue.21 |s2cid=100097624 }}</ref><ref>{{cite journal |author=Nikonov, M. V. |author2=Bessonov, A. A. |author3=Krot, N. N. |author4=Perminov, V. P. |date=1994 |title=Synthesis and characterization of Np(VI, VII) mixed-valence compound |journal=Radiochemistry |volume=36 | pages=237–8}}</ref>

===Oxides===
Three anhydrous neptunium oxides have been reported, [[neptunium(IV) oxide|NpO<sub>2</sub>]], Np<sub>2</sub>O<sub>5</sub>, and Np<sub>3</sub>O<sub>8</sub>, though some studies<ref>{{cite journal |author=Fahey, J. A. |author2=Turcotte, R. P. |author3=Chikalla, T. D. |date=1976 |title=Decomposition, stoichiometry and structure of neptunium oxides |journal=Journal of Inorganic and Nuclear Chemistry |volume=38 |issue=3 |pages=495–500 |doi=10.1016/0022-1902(76)80291-6}}</ref> have stated that only the first two of these exist, suggesting that claims of Np<sub>3</sub>O<sub>8</sub> are actually the result of mistaken analysis of Np<sub>2</sub>O<sub>5</sub>. However, as the full extent of the reactions that occur between neptunium and oxygen has yet to be researched, it is not certain which of these claims is accurate. Although neptunium oxides have not been produced with neptunium in oxidation states as high as those possible with the adjacent actinide uranium, neptunium oxides are more stable at lower oxidation states. This behavior is illustrated by the fact that NpO<sub>2</sub> can be produced by simply burning neptunium salts of oxyacids in air.<ref name="lanl" /><ref name="Yoshida724">Yoshida et al., 724–726.</ref><ref name="sharma">{{cite book |last=Sharma |first=B. K. |title=Nuclear and Radiation Chemistry - Transuranium elements |publisher=Krishna Prakashan Media|pages=128–9| url=https://books.google.com/books?id=L8mBZcaGUQAC|isbn=9788185842639 }}</ref><ref name="Richteretal">{{cite journal |author=Richter K. |author2= Sari C. |date=1987 |title=Phase relationships in the neptunium-oxygen system |journal=Journal of Nuclear Materials |volume=148 |issue=3 | pages=266–71|bibcode = 1987JNuM..148..266R |doi = 10.1016/0022-3115(87)90019-5 }}</ref>

The greenish-brown NpO<sub>2</sub> is very stable over a large range of pressures and temperatures and does not undergo [[phase transition]]s at low temperatures. It does show a phase transition from face-centered cubic to orthorhombic at around 33–37&nbsp;GPa, although it returns to its original phase when pressure is released. It remains stable under oxygen pressures up to 2.84&nbsp;MPa and temperatures up to 400&nbsp;°C. 
Np<sub>2</sub>O<sub>5</sub> is black-brown in color and [[monoclinic]] with a lattice size of 418×658×409&nbsp;picometres. It is relatively unstable and decomposes to NpO<sub>2</sub> and O<sub>2</sub> at 420–695&nbsp;°C. Although Np<sub>2</sub>O<sub>5</sub> was initially subject to several studies that claimed to produce it with mutually contradictory methods, it was eventually prepared successfully by heating neptunium [[peroxide]] to 300–350&nbsp;°C for 2–3 hours or by heating it under a layer of water in an [[ampoule]] at 180&nbsp;°C.<ref name="Yoshida724" /><ref name="Richteretal" /><ref>{{cite journal |author=Benedict, U. |author2=Dabos, S. |author3=Dufour, C. |author4=Spirelet, J. C. |date=1986 |title=Neptunium compounds under high pressure |journal=Journal of the Less Common Metals |volume=121 | pages=461–68 |doi=10.1016/0022-5088(86)90563-1}}</ref><ref>{{cite book|first1 = J. A.|last1 = Fahey|contribution = Neptunium|title = The Chemistry of the Actinide Elements|editor1-first = J. J.|editor1-last = Katz|editor2-first = G. T.|editor2-last = Seaborg|editor3-first = L. R.|editor3-last = Morss|date = 1986|volume = 1|publisher = Chapman & Hall|location = New York|page = 456}}</ref>

Neptunium also forms a large number of oxide compounds with a wide variety of elements, although the neptunate oxides formed with [[alkali metal]]s and [[alkaline earth metals]] have been by far the most studied. Ternary neptunium oxides are generally formed by reacting NpO<sub>2</sub> with the oxide of another element or by precipitating from an alkaline solution. [[Lithium|Li]]<sub>5</sub>NpO<sub>6</sub> has been prepared by reacting Li<sub>2</sub>O and NpO<sub>2</sub> at 400&nbsp;°C for 16 hours or by reacting Li<sub>2</sub>O<sub>2</sub> with NpO<sub>3</sub> · H<sub>2</sub>O at 400&nbsp;°C for 16 hours in a quartz tube and flowing oxygen. Alkali neptunate compounds [[Potassium|K]]<sub>3</sub>NpO<sub>5</sub>, [[Caesium|Cs]]<sub>3</sub>NpO<sub>5</sub>, and [[Rubidium|Rb]]<sub>3</sub>NpO<sub>5</sub> are all produced by a similar reaction: 
:NpO<sub>2</sub> + 3 MO<sub>2</sub> → M<sub>3</sub>NpO<sub>5</sub>&nbsp;(M = K, Cs, Rb)

The oxide compounds KNpO<sub>4</sub>, CsNpO<sub>4</sub>, and RbNpO<sub>4</sub> are formed by reacting Np(VII) ({{chem|[NpO|4|(OH)|2|]|3-}}) with a compound of the alkali metal [[nitrate]] and [[ozone]]. Additional compounds have been produced by reacting NpO<sub>3</sub> and water with solid alkali and alkaline [[peroxide]]s at temperatures of 400–600&nbsp;°C for 15–30 hours. Some of these include Ba<sub>3</sub>(NpO<sub>5</sub>)<sub>2</sub>, Ba<sub>2</sub>[[Sodium|Na]]NpO<sub>6</sub>, and Ba<sub>2</sub>LiNpO<sub>6</sub>. Also, a considerable number of hexavalent neptunium oxides are formed by reacting solid-state NpO<sub>2</sub> with various alkali or alkaline earth oxides in an environment of flowing oxygen. Many of the resulting compounds also have an equivalent compound that substitutes uranium for neptunium. Some compounds that have been characterized include Na<sub>2</sub>Np<sub>2</sub>O<sub>7</sub>, Na<sub>4</sub>NpO<sub>5</sub>, Na<sub>6</sub>NpO<sub>6</sub>, and Na<sub>2</sub>NpO<sub>4</sub>. These can be obtained by heating different combinations of NpO<sub>2</sub> and Na<sub>2</sub>O to various temperature thresholds and further heating will also cause these compounds to exhibit different neptunium allotropes. The lithium neptunate oxides Li<sub>6</sub>NpO<sub>6</sub> and Li<sub>4</sub>NpO<sub>5</sub> can be obtained with similar reactions of NpO<sub>2</sub> and Li<sub>2</sub>O.<ref name="Yoshida728">Yoshida et al, pp. 728–730.</ref><ref>{{cite journal |title=Die reaktion der oxide der transurane mit alkalioxiden—I: Ternäre oxide der sechswertigen transurane mit lithium und natrium |author=Keller, C. |author2=Kock, L. |author3=Walter, K. H. |date=1965 |journal=Journal of Inorganic and Nuclear Chemistry |volume=27 |issue=6 | pages=1205–23 |doi=10.1016/0022-1902(65)80083-5}}</ref><ref>{{cite journal |author=Carnall, W. T. |author2=Neufeldt, S. J. |author3=Walker, A. |date=1965 |title=Reactions in Molten Salt Solutions. I. Uranate and Neptunate Formation in Molten Lithium Nitrate-Sodium Nitrate |journal=Inorganic Chemistry |volume=4 |issue=12 | pages=1808–13| doi=10.1021/ic50034a034}}</ref><ref>{{cite journal |author=Morss, L. R. |author2=Appelman, E. H. |author3=Gerz, R. R. |author4=Martin-Rovet, D. |date=1994 |title=Structural studies of Li<sub>5</sub>ReO<sub>6</sub>, Li<sub>4</sub>NpO<sub>5</sub> and Li<sub>5</sub>NpO<sub>6</sub> by neutron and X-ray powder diffraction |journal=Journal of Alloys and Compounds |volume=203 | pages=289–95|doi=10.1016/0925-8388(94)90748-x |url=https://zenodo.org/record/1258645 }}</ref><ref>{{cite journal |title=Li<sub>5</sub>NpO<sub>6</sub>, die erste kristalline verbindung mit siebenwertigem neptunium; zur frage der existenz von siebenwertigem plutonium und americium |author=Keller, C. |author2=Seiffert, H. |date=1969 |journal=Inorganic and Nuclear Chemistry Letters |volume=5 | pages=51–7 |doi=10.1016/0020-1650(69)80236-9}}</ref><ref>{{cite journal |author=Awasthi, S. K. |author2=Martinot, L. |author3=Fuger, J. |author4=Duyckaerts, G. |date=1971 |title=Preparation and Characterisation of some Np(VII) Compounds |journal=Inorganic and Nuclear Chemistry Letters |volume=7 |issue=2 | pages=145–51 |doi=10.1016/0020-1650(71)80143-5}}</ref><ref>{{cite journal |author=Pagès, M. |author2=Nectoux, F. |author3=Freundlich, W. |date=1971 |journal=Radiochemical and Radioanalytical Letters |volume=7 | pages=155–62}}</ref><ref>{{cite journal |author=Mefod'eva, M. P. |author2=Krot, N. N. |author3=Smirnova, T. V. |author4=Gel'man, A. D. |date=1969 |title=Oxalate Compounds of Hexavalent Neptunium |language=ru |journal=Radiokhimiya |volume=11 | pages=193–200}}</ref>

A large number of additional alkali and alkaline neptunium oxide compounds such as Cs<sub>4</sub>Np<sub>5</sub>O<sub>17</sub> and Cs<sub>2</sub>Np<sub>3</sub>O<sub>10</sub> have been characterized with various production methods. Neptunium has also been observed to form ternary oxides with many additional elements in [[Group (periodic table)|group]]s 3 through 7, although these compounds are much less well studied.<ref name="Yoshida728" /><ref>{{cite journal |title=Some ternary oxides of neptunium and plutonium with the alkali metals |author=Hoekstra, H. R. |author2=Gebert, E. |date=1977 |journal=Journal of Inorganic and Nuclear Chemistry |volume=39 |issue=12 | pages=2219–21 |doi=10.1016/0022-1902(77)80399-0}}</ref><ref>{{cite book|first1 = A.|last1 = Tabuteau| first2=M.|last2=Pagès|contribution = Neptunium |title = Handbook on the Physics and Chemistry of the Actinides |editor1-first = A. J.|editor1-last = Freeman|editor2-first = C.|editor2-last = Keller|date = 1985|volume = 3|publisher = North-Holland|location = Amsterdam|pages = 184–241}}</ref>

===Halides===
{{further|Neptunium hexafluoride}}
Although neptunium [[halide]] compounds have not been nearly as well studied as its oxides, a fairly large number have been successfully characterized. Of these, neptunium [[fluoride]]s have been the most extensively researched, largely because of their potential use in separating the element from nuclear waste products. Four binary neptunium fluoride compounds, Np[[fluorine|F]]<sub>3</sub>, NpF<sub>4</sub>, NpF<sub>5</sub>, and NpF<sub>6</sub>, have been reported. The first two are fairly stable and were first prepared in 1947{{Contradictory inline|date=December 2024|reason=If NpF6 was first prepared in 1943 from NpF3, then NpF3 should be discovered earlier than NpF6.}} through the following reactions:{{Citation needed|date=December 2024}}
:NpO<sub>2</sub> + {{frac|1|2}} H<sub>2</sub> + 3 HF → NpF<sub>3</sub> + 2 H<sub>2</sub>O&nbsp;&nbsp;&nbsp;(400°C)
:NpF<sub>3</sub> + {{frac|1|2}} O<sub>2</sub> + HF → NpF<sub>4</sub> + {{frac|1|2}} H<sub>2</sub>O&nbsp;&nbsp;(400°C)
Later, NpF<sub>4</sub> was obtained directly by heating NpO<sub>2</sub> to various temperatures in mixtures of either [[hydrogen fluoride]] or pure fluorine gas. NpF<sub>5</sub> is much more difficult to form and most known preparation methods involve reacting NpF<sub>4</sub> or NpF<sub>6</sub> compounds with various other fluoride compounds. NpF<sub>5</sub> will decompose into NpF<sub>4</sub> and NpF<sub>6</sub> when heated to around 320&nbsp;°C.<ref>S. Fried, N. R. Davidson: ''[https://apps.dtic.mil/sti/pdfs/ADA315026.pdf The Basic Dry Chemistry of Neptunium]'', (1947) Report MDDC-1332, United States Atomic Energy Commission - Argonne National Laboratory, Declassified: July 18, 1947.</ref><ref name="Yoshida730">Yoshida et al, pp. 730–736.</ref><ref>{{cite journal |author=Kleinschmidt, P. D. |author2=Lau, K. H. |author3=Hildenbrand, D. L. |date=1992 |title=Sublimation studies of NpF<sub>4</sub> |journal=Journal of Chemical Physics |volume=97 |issue=3 | pages=1950–3|bibcode = 1992JChPh..97.1950K |doi = 10.1063/1.463131 }}</ref><ref name="Drobyetal">{{cite journal |author=Drobyshevskii, Y. V. |author2=Serik, V. F. |author3=Sokolov, V. B. |author4=Tul'skii, M. N. |date=1978 |title=Synthesis and some properties of neptunium oxide tetrafluoride and neptunium pentafluoride |journal=Radiokhimiya |volume=20 | pages=238–243 |language=ru}}</ref>

NpF<sub>6</sub> or [[neptunium hexafluoride]] is extremely volatile, as are its adjacent actinide compounds [[uranium hexafluoride]] (UF<sub>6</sub>) and [[plutonium hexafluoride]] (PuF<sub>6</sub>). This volatility has attracted a large amount of interest to the compound in an attempt to devise a simple method for extracting neptunium from spent nuclear power station fuel rods. NpF<sub>6</sub> was first prepared in 1943 by reacting NpF<sub>3</sub> and gaseous fluorine at very high temperatures and the first bulk quantities were obtained in 1958 by heating NpF<sub>4</sub> and dripping pure fluorine on it in a specially prepared apparatus. Additional methods that have successfully produced neptunium hexafluoride include reacting [[bromine trifluoride|BrF<sub>3</sub>]] and [[bromine pentafluoride|BrF<sub>5</sub>]] with NpF<sub>4</sub> and by reacting several different neptunium oxide and fluoride compounds with anhydrous hydrogen fluorides.<ref name="Yoshida730" /><ref>Seaborg, G. T. and Brown, H. S. (1961) [https://patents.google.com/patent/US2982604 US Patent No. 2,982,604].</ref><ref>Florin, A. E. (1943) Report MUC-GTS-2165, Declassified: January 23, 1946.</ref><ref>{{cite journal |author=Malm, J. G. |author2=Weinstock, B. |author3=Weaver, E. |date=1958 |title=The Preparation and Properties of NpF<sub>6</sub>; a Comparison with PuF<sub>6</sub> |journal=Journal of Physical Chemistry |volume=62 |issue=12 | pages=1506–8| doi=10.1021/j150570a009}}</ref>

Four neptunium [[Oxohalide|oxyfluoride]] compounds, NpO<sub>2</sub>F, NpOF<sub>3</sub>, NpO<sub>2</sub>F<sub>2</sub>, and NpOF<sub>4</sub>, have been reported, although none of them have been extensively studied. NpO<sub>2</sub>F<sub>2</sub> is a pinkish solid and can be prepared by reacting NpO<sub>3</sub> · H<sub>2</sub>O and Np<sub>2</sub>F<sub>5</sub> with pure fluorine at around 330&nbsp;°C. NpOF<sub>3</sub> and NpOF<sub>4</sub> can be produced by reacting neptunium oxides with anhydrous hydrogen fluoride at various temperatures. Neptunium also forms a wide variety of fluoride compounds with various elements. Some of these that have been characterized include CsNpF<sub>6</sub>, Rb<sub>2</sub>NpF<sub>7</sub>, Na<sub>3</sub>NpF<sub>8</sub>, and K<sub>3</sub>NpO<sub>2</sub>F<sub>5</sub>.<ref name="Yoshida730" /><ref name="Drobyetal" /><ref>{{cite book|first1 = S.|last1 = Fried|contribution = Neptunium |title = The Actinide Elements |editor1-first = G. T.|editor1-last = Seaborg|editor2-first = J. J.|editor2-last = Katz|date = 1954 |publisher = McGraw-Hill|location = New York|page = 471}}</ref><ref>{{cite journal |author=Eller, P. G. |display-authors=4 |author2=Asprey, L. B. |author3=Kinkead, S. A. |author4=Swanson, B. I. |author5=Kissane, R. J. |date=1998 |title=Reaction of Dioxygen Difluoride with Neptunium Oxides and Fluorides |journal=Journal of Alloys and Compounds |volume=269 |issue=1–2 | pages=63–6 |doi=10.1016/s0925-8388(98)00005-x|url=https://zenodo.org/record/1260147 }}</ref><ref>{{cite journal |author=Kleinschmidt, P. D. |author2=Lau, K. H. |author3=Hildenbrand, D. L. |date=1992 |title=Sublimation studies of NpO<sub>2</sub>F<sub>2</sub> |journal=Journal of Chemical Physics |volume=97 |issue=4 | pages=2417–21|doi=10.1063/1.463080 |bibcode=1992JChPh..97.2417K }}</ref><ref>{{cite journal |author=Peacock, R. D. |author2=Edelstein, N. |date=1976 |title=Some reactions of neptunium hexafluoride |journal=Journal of Inorganic and Nuclear Chemistry |volume=38 |issue=4 | pages=771–3 |doi=10.1016/0022-1902(76)80353-3|url=http://www.escholarship.org/uc/item/1m52k460 }}</ref><ref>{{cite book|first1 = D.|last1 = Brown|contribution = Transuranium |title = Gmelin Handbuck der Anorganischen Chemie, Suppl. Work |date = 1972|volume = 4|publisher = Verlag Chemie|location = Weinheim, Germany}}</ref>

Two neptunium [[chloride]]s, Np[[chlorine|Cl]]<sub>3</sub> and NpCl<sub>4</sub>, have been characterized. Although several attempts to obtain NpCl<sub>5</sub> have been made, they have not been successful. NpCl<sub>3</sub> is produced by reducing neptunium dioxide with hydrogen and [[carbon tetrachloride]] ([[carbon|C]]Cl<sub>4</sub>) and NpCl<sub>4</sub> by reacting a neptunium oxide with CCl<sub>4</sub> at around 500&nbsp;°C. Other neptunium chloride compounds have also been reported, including NpOCl<sub>2</sub>, Cs<sub>2</sub>NpCl<sub>6</sub>, Cs<sub>3</sub>NpO<sub>2</sub>Cl<sub>4</sub>, and Cs<sub>2</sub>NaNpCl<sub>6</sub>. Neptunium [[bromide]]s Np[[bromine|Br]]<sub>3</sub> and NpBr<sub>4</sub> have also been produced; the latter by reacting [[aluminium bromide]] with NpO<sub>2</sub> at 350&nbsp;°C and the former in an almost identical procedure but with [[zinc]] present. The neptunium [[iodide]] Np[[iodine|I]]<sub>3</sub> has also been prepared by the same method as NpBr<sub>3</sub>.<ref name="Yoshida736">Yoshida et al, pp. 736–738.</ref><ref>Fried, S. and Davidson, N. R. (1951) [https://patents.google.com/patent/US2578416 US Patent No. 2,578,416].</ref><ref>Lemire, pp. 143–155.</ref>

===Chalcogenides, pnictides, and carbides===
Neptunium [[chalcogen]] and [[pnictogen]] compounds have been well studied primarily as part of research into their electronic and magnetic properties and their interactions in the natural environment. Pnictide and [[carbide]] compounds have also attracted interest because of their presence in the fuel of several advanced nuclear reactor designs, although the latter group has not had nearly as much research as the former.<ref name="Yoshida739">Yoshida et al, pp. 739–742.</ref>

;Chalcogenides
A wide variety of neptunium [[sulfide]] compounds have been characterized, including the pure sulfide compounds Np[[sulfur|S]], NpS<sub>3</sub>, Np<sub>2</sub>S<sub>5</sub>, Np<sub>3</sub>S<sub>5</sub>, Np<sub>2</sub>S<sub>3</sub>, and Np<sub>3</sub>S<sub>4</sub>. Of these, Np<sub>2</sub>S<sub>3</sub>, prepared by reacting NpO<sub>2</sub> with [[hydrogen sulfide]] and [[carbon disulfide]] at around 1000&nbsp;°C, is the most well-studied and three allotropic forms are known. The α form exists up to around 1230&nbsp;°C, the β up to 1530&nbsp;°C, and the γ form, which can also exist as Np<sub>3</sub>S<sub>4</sub>, at higher temperatures. NpS can be produced by reacting Np<sub>2</sub>S<sub>3</sub> and neptunium metal at 1600&nbsp;°C and Np<sub>3</sub>S<sub>5</sub> can be prepared by the decomposition of Np<sub>2</sub>S<sub>3</sub> at 500&nbsp;°C or by reacting sulfur and neptunium hydride at 650&nbsp;°C. Np<sub>2</sub>S<sub>5</sub> is made by heating a mixture of Np<sub>3</sub>S<sub>5</sub> and pure sulfur to 500&nbsp;°C. All of the neptunium sulfides except for the β and γ forms of Np<sub>2</sub>S<sub>3</sub> are [[isostructural]] with the equivalent uranium sulfide and several, including NpS, α−Np<sub>2</sub>S<sub>3</sub>, and β−Np<sub>2</sub>S<sub>3</sub> are also isostructural with the equivalent plutonium sulfide. The oxysulfides NpOS, Np<sub>4</sub>O<sub>4</sub>S, and Np<sub>2</sub>O<sub>2</sub>S have also been produced, although the latter three have not been well studied. NpOS was first prepared in 1985 by vacuum sealing NpO<sub>2</sub>, Np<sub>3</sub>S<sub>5</sub>, and pure sulfur in a quartz tube and heating it to 900&nbsp;°C for one week.<ref name="Yoshida739" /><ref>Marcon, J. P. (1967) C. R. Acad. Sci. Paris 265 Series C 235.</ref><ref>{{cite journal |author=Fried, S. |author2=Davidson, N. R. |date=1948 |title= The preparation of solid neptunium compounds|journal=Journal of the American Chemical Society |volume=70 |issue=11 | pages=3539–47|pmid=18102891 |doi=10.1021/ja01191a003|bibcode=1948JAChS..70.3539F }}</ref><ref>{{cite journal |author=Zachariasen, W. H. |date=1949 |title=Crystal chemical studies of the tf-series of elements. X. Sulfides and ox\-sulfides |journal=Acta Crystallographica |volume=2 |issue=5 | pages=291–6 |doi=10.1107/s0365110x49000758|doi-access=free |bibcode=1949AcCry...2..291Z }}</ref><ref>{{cite book|author=Charvillat, J. P.|display-authors=4|author2=Benedict, U.|author3=Damien, D.|author4=Novion, D.|author5=Wojakowski, A.|author6=Muller, W. |contribution = Neptunium |title = Tranplutonium Elements |editor1-first = W.|editor1-last = Muller |editor2-first = R.|editor2-last = Linder|date = 1976|publisher = North-Holland|location = Amsterdam|page = 79}}</ref><ref>{{cite journal |author=Peacock, R. D. |author2=Edelstein, N. |date=1997 |title=High pressure X-ray diffraction experiments on NpS and PuS up to 60 GPa |journal=High Pressure Research |volume=15 |issue=6 | pages=387–92|bibcode = 1997HPR....15..387B |doi = 10.1080/08957959708240482 }}</ref><ref name="Pages1985">{{cite journal |author=Thevenin, T. |author2=Jove, J. |author3=Pages, M. |date=1985 |title=Crystal chemistry and <sup>237</sup>Np mossbauer investigations on neptunium oxide chalcogenides NpOS and NpOSe |journal=Materials Research Bulletin |volume=20 |issue=6 | pages=723–30 |doi=10.1016/0025-5408(85)90151-5}}</ref>

Neptunium [[selenide]] compounds that have been reported include Np[[selenium|Se]], NpSe<sub>3</sub>, Np<sub>2</sub>Se<sub>3</sub>, Np<sub>2</sub>Se<sub>5</sub>, Np<sub>3</sub>Se<sub>4</sub>, and Np<sub>3</sub>Se<sub>5</sub>. All of these have only been obtained by heating neptunium hydride and selenium metal to various temperatures in a vacuum for an extended period of time and Np<sub>2</sub>Se<sub>3</sub> is only known to exist in the γ allotrope at relatively high temperatures. Two neptunium [[oxyselenide]] compounds are known, NpOSe and Np<sub>2</sub>O<sub>2</sub>Se, are formed with similar methods by replacing the neptunium hydride with neptunium dioxide. The known neptunium [[telluride (chemistry)|telluride]] compounds Np[[tellurium|Te]], NpTe<sub>3</sub>, Np<sub>3</sub>Te<sub>4</sub>, Np<sub>2</sub>Te<sub>3</sub>, and Np<sub>2</sub>O<sub>2</sub>Te are formed by similar procedures to the selenides and Np<sub>2</sub>O<sub>2</sub>Te is isostructural to the equivalent uranium and plutonium compounds. No neptunium−[[polonium]] compounds have been reported.<ref name="Yoshida739" /><ref name="Pages1985" /><ref>{{cite book|author=Damien, D.|author2=Berger, R. |title = Supplement to Journal of Inorganic and Nuclear Chemistry - Moscow Symposium - On the Chemistry of Transuranium Elements |editor1-first = V. I.|editor1-last = Spitsyn |editor2-first = J. J.|editor2-last = Katz|date = 1976|publisher = Pergamon Press|location = Oxford|pages = 109–16}}</ref><ref>{{cite journal |author=Thevenin, T. |author2=Pages, M. |date=1982 |title=Crystallographic and magnetic studies of a new neptunium selenide: Np<sub>2</sub>Se<sub>5</sub> |journal=Journal of the Less Common Metals |volume=84 | pages=133–7 |doi=10.1016/0022-5088(82)90138-2}}</ref><ref>{{cite journal |author=Damien, D. |author2=Wojakowski, A. |date=1975 |title=Preparation et parametres de maille des monoseleniures et monotellurures de neptunium et d'americium |journal=Radiochemical and Radioanalytical Letters |volume=23 | pages=145–54|language = fr}}</ref>

;Pnictides and carbides
Neptunium [[nitride]] (Np[[nitrogen|N]]) was first prepared in 1953 by reacting neptunium hydride and [[ammonia]] gas at around 750&nbsp;°C in a quartz capillary tube. Later, it was produced by reacting different mixtures of nitrogen and hydrogen with neptunium metal at various temperatures. It has also been produced by the reduction of neptunium dioxide with [[Diatomic molecule|diatomic]] nitrogen gas at 1550&nbsp;°C. NpN is [[Isomorphism (crystallography)|isomorphous]] with [[Uranium nitride|uranium mononitride]] (UN) and [[Plutonium nitride|plutonium mononitride]] (PuN) and has a melting point of 2830&nbsp;°C under a nitrogen pressure of around 1 MPa. Two neptunium [[phosphide]] compounds have been reported, Np[[phosphorus|P]] and Np<sub>3</sub>P<sub>4</sub>. The first has a face centered cubic structure and is prepared by converting neptunium metal to a powder and then reacting it with [[phosphine]] gas at 350&nbsp;°C. Np<sub>3</sub>P<sub>4</sub> can be produced by reacting neptunium metal with [[red phosphorus]] at 740&nbsp;°C in a vacuum and then allowing any extra phosphorus to [[sublimation (phase transition)|sublimate]] away. The compound is non-reactive with water but will react with [[nitric acid]] to produce Np(IV) solution.<ref name="Yoshida742">Yoshida et al, pp. 742–744.</ref><ref name="fried1953">{{cite journal |author=Sheft, I. |author2=Fried, S. |date=1953 |title=Neptunium Compounds |journal=Journal of the American Chemical Society |volume=75 |issue=5 | pages=1236–7 |doi=10.1021/ja01101a067|bibcode=1953JAChS..75.1236S }}</ref><ref>{{cite journal |author=Olson, W. M. |author2=Mulford R. N. R. |date=1966 |title=The melting point and decomposition pressure of neptunium mononitride |journal=Journal of Physical Chemistry |volume=70 |issue=9 | pages=2932–2934 |doi=10.1021/j100881a035}}</ref>

Three neptunium [[arsenide]] compounds have been prepared, Np[[arsenic|As]], NpAs<sub>2</sub>, and Np<sub>3</sub>As<sub>4</sub>. The first two were first produced by heating arsenic and neptunium hydride in a vacuum-sealed tube for about a week. Later, NpAs was also made by confining neptunium metal and arsenic in a vacuum tube, separating them with a quartz membrane, and heating them to just below neptunium's melting point of 639&nbsp;°C, which is slightly higher than the arsenic's sublimation point of 615&nbsp;°C. Np<sub>3</sub>As<sub>4</sub> is prepared by a similar procedure using iodine as a [[Chemical transport reaction|transporting agent]]. NpAs<sub>2</sub> crystals are brownish gold and Np<sub>3</sub>As<sub>4</sub> is black. The neptunium [[antimonide]] compound Np[[antimony|Sb]] was produced in 1971 by placing equal quantities of both elements in a vacuum tube, heating them to the melting point of antimony, and then heating it further to 1000&nbsp;°C for sixteen days. This procedure also produced trace amounts of an additional antimonide compound Np<sub>3</sub>Sb<sub>4</sub>. One neptunium-[[bismuth]] compound, NpBi, has also been reported.<ref name="Yoshida742" /><ref name="fried1953" /><ref>{{cite journal |author=Blaise, A. |author2=Damien, D. |author3=Suski, W. |date=1981 |title=Electrical resistivity of neptunium mono and diarsenide |journal=Solid State Communications |volume=37 |issue=8 | pages=659–62|bibcode = 1981SSCom..37..659B |doi = 10.1016/0038-1098(81)90543-3 }}</ref><ref>{{cite journal |author=Dabos, S. |display-authors=4 |author2=Dufour, C. |author3=Benedict, U. |author4=Spirlet, J. C. |author5=Pages, M. |date=1986 |title=High-pressure X-ray diffraction on neptunium compounds: Recent results for NpAs |journal=Physica B |volume=144 |issue=1 | pages=79–83 |doi=10.1016/0378-4363(86)90296-2|bibcode=1986PhyBC.144...79D }}</ref><ref>{{cite journal |author=Aldred, A. T. |display-authors=4 |author2=Dunlap, B. D. |author3=Harvey, A. R. |author4=Lam, D. J. |author5=Lander, G. H. |author6=Mueller, M. H. |date=1974 |title= Magnetic properties of the neptunium monopnictides|journal=Physical Review B |volume=9 |issue=9 | pages=3766–78|bibcode = 1974PhRvB...9.3766A |doi = 10.1103/PhysRevB.9.3766 }}</ref><ref>{{cite journal |author=Burlet, P. |display-authors=4 |author2=Bourdarot, F. |author3=Rossat-Mignod, J. |author4=Sanchez, J. P. |author5=Spirlet, J. C. |author6=Rebizant, J. |author7=Vogt, O. |date=1992 |title=Neutron diffraction study of the magnetic ordering in NpBi |journal=Physica B |volume=180 | pages=131–2|bibcode=1992PhyB..180..131B |doi=10.1016/0921-4526(92)90683-J }}</ref>

The neptunium [[carbide]]s Np[[carbon|C]], Np<sub>2</sub>C<sub>3</sub>, and NpC<sub>2</sub> (tentative) have been reported, but have not characterized in detail despite the high importance and utility of actinide carbides as advanced nuclear reactor fuel. NpC is a [[non-stoichiometric compound]], and could be better labelled as NpC<sub>''x''</sub> (0.82 ≤ ''x'' ≤ 0.96). It may be obtained from the reaction of neptunium hydride with [[graphite]] at 1400&nbsp;°C or by heating the constituent elements together in an [[electric arc furnace]] using a [[tungsten]] electrode. It reacts with excess carbon to form pure Np<sub>2</sub>C<sub>3</sub>. NpC<sub>2</sub> is formed from heating NpO<sub>2</sub> in a graphite crucible at 2660–2800&nbsp;°C.<ref name="Yoshida742" /><ref name="fried1953" /><ref>{{cite journal |author=De Novion, C. H. |author2=Lorenzelli, R. |date=1968 |title=Proprietes electroniques du monocarbure et du mononitrure de neptunium |journal=Journal of Physics and Chemistry of Solids |volume=29 |issue=10 | pages=1901–5|bibcode =  1968JPCS...29.1901D|doi = 10.1016/0022-3697(68)90174-1 }}</ref><ref>{{cite book |author= Holley, C. E. Jr. |author2= Rand, M. H. |author3= Storms, E. K. |title=The Chemical Thermodynamics of Actinide Elements and Compounds, part 6. The Actinide Carbides|publisher=International Atomic Energy Agency|location=Vienna | pages=49–51|date=1984}}</ref>

===Other inorganic===
;Hydrides
Neptunium reacts with [[hydrogen]] in a similar manner to its neighbor plutonium, forming the [[hydride]]s NpH<sub>2+''x''</sub> ([[face-centered cubic]]) and NpH<sub>3</sub> ([[hexagonal crystal system|hexagonal]]). These are [[isostructural]] with the corresponding plutonium hydrides, although unlike PuH<sub>2+''x''</sub>, the [[lattice parameter]]s of NpH<sub>2+''x''</sub> become greater as the hydrogen content (''x'') increases. The hydrides require extreme care in handling as they decompose in a vacuum at 300&nbsp;°C to form finely divided neptunium metal, which is [[pyrophoric]].<ref name="Yoshida722">Yoshida et al., pp. 722–4.</ref>

;Phosphates, sulfates, and carbonates
Being chemically stable, neptunium [[phosphate]]s have been investigated for potential use in immobilizing nuclear waste. Neptunium pyrophosphate (α-NpP<sub>2</sub>O<sub>7</sub>), a green solid, has been produced in the reaction between neptunium dioxide and [[boron phosphate]] at 1100&nbsp;°C, though neptunium(IV) phosphate has so far remained elusive. The series of compounds NpM<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>, where M is an [[alkali metal]] ([[lithium|Li]], [[sodium|Na]], [[potassium|K]], [[rubidium|Rb]], or [[caesium|Cs]]), are all known. Some neptunium [[sulfate]]s have been characterized, both aqueous and solid and at various oxidation states of neptunium (IV through VI have been observed). Additionally, neptunium [[carbonate]]s have been investigated to achieve a better understanding of the behavior of neptunium in [[geological repository|geological repositories]] and the environment, where it may come into contact with carbonate and [[bicarbonate]] aqueous solutions and form soluble complexes.<ref>Lemire et al., pp. 177–180.</ref><ref name="Yoshida744">Yoshida et al., pp. 744–5.</ref>

===Organometallic===
[[File:Neptunocene-from-xtal-3D-balls.png|thumb|upright=1.2|Structure of neptunocene]]
A few organoneptunium compounds are known and chemically characterized, although not as many as for [[organouranium compound|uranium]] due to neptunium's scarcity and radioactivity. The most well known organoneptunium compounds are the [[cyclopentadienyl]] and [[cyclooctatetraenyl]] compounds and their derivatives.<ref name="Yoshida750">Yoshida et al., pp. 750–2.</ref> The trivalent cyclopentadienyl compound Np(C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>·[[tetrahydrofuran|THF]] was obtained in 1972 from reacting Np(C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>Cl with [[sodium]], although the simpler Np(C<sub>5</sub>H<sub>5</sub>) could not be obtained.<ref name="Yoshida750" /> Tetravalent neptunium cyclopentadienyl, a reddish-brown complex, was synthesized in 1968 by reacting neptunium(IV) chloride with potassium cyclopentadienide:<ref name="Yoshida750" />

:NpCl<sub>4</sub> + 4&nbsp;KC<sub>5</sub>H<sub>5</sub> → Np(C<sub>5</sub>H<sub>5</sub>)<sub>4</sub> + 4&nbsp;KCl

It is soluble in [[benzene]] and [[Tetrahydrofuran|THF]], and is less sensitive to [[oxygen]] and water than [[plutonium|Pu]](C<sub>5</sub>H<sub>5</sub>)<sub>3</sub> and [[americium|Am]](C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>.<ref name="Yoshida750" /> Other Np(IV) cyclopentadienyl compounds are known for many [[ligand]]s: they have the general formula (C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>NpL, where L represents a ligand.<ref name="Yoshida750" />
[[Neptunocene]], Np(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>, was synthesized in 1970 by reacting neptunium(IV) chloride with K<sub>2</sub>(C<sub>8</sub>H<sub>8</sub>). It is [[Isomorphism (crystallography)|isomorphous]] to [[uranocene]] and [[plutonocene]], and they behave chemically identically: all three compounds are insensitive to water or dilute bases but are sensitive to air, reacting quickly to form oxides, and are only slightly soluble in benzene and [[toluene]].<ref name="Yoshida750" /> Other known neptunium cyclooctatetraenyl derivatives include Np(RC<sub>8</sub>H<sub>7</sub>)<sub>2</sub> (R = [[ethanol]], [[butanol]]) and KNp(C<sub>8</sub>H<sub>8</sub>)·2THF, which is isostructural to the corresponding plutonium compound.<ref name="Yoshida750" /> In addition, neptunium [[hydrocarbyl]]s have been prepared, and solvated triiodide complexes of neptunium are a precursor to many organoneptunium and inorganic neptunium compounds.<ref name="Yoshida750" />

===Coordination complexes===
There is much interest in the [[coordination chemistry]] of neptunium, because its five oxidation states all exhibit their own distinctive chemical behavior, and the coordination chemistry of the actinides is heavily influenced by the [[actinide contraction]] (the greater-than-expected decrease in [[ionic radii]] across the actinide series, analogous to the [[lanthanide contraction]]).<ref name="Yoshida745">Yoshida et al., pp. 745–750.</ref>

====Solid state====
Few neptunium(III) coordination compounds are known, because Np(III) is readily oxidized by atmospheric oxygen while in aqueous solution. However, [[sodium formaldehyde sulfoxylate]] can reduce Np(IV) to Np(III), stabilizing the lower oxidation state and forming various sparingly soluble Np(III) coordination complexes, such as {{chem|Np|2|(C|2|O|4|)|3}}·11H<sub>2</sub>O, {{chem|Np|2|(C|6|H|5|AsO|3|)|3}}·H<sub>2</sub>O, and {{chem|Np|2|[C|6|H|4|(OH)COO]|3}}.<ref name="Yoshida745" />

Many neptunium(IV) coordination compounds have been reported, the first one being {{chem|(Et|4|N)Np(NCS)|8}}, which is isostructural with the analogous uranium(IV) coordination compound.<ref name="Yoshida745" /> Other Np(IV) coordination compounds are known, some involving other metals such as [[cobalt]] ({{chem|CoNp|2|F|10}}·8H<sub>2</sub>O, formed at 400&nbsp;K) and [[copper]] ({{chem|CuNp|2|F|10}}·6H<sub>2</sub>O, formed at 600&nbsp;K).<ref name="Yoshida745" /> Complex nitrate compounds are also known: the experimenters who produced them in 1986 and 1987 obtained single crystals by slow evaporation of the Np(IV) solution at ambient temperature in concentrated [[nitric acid]] and excess 2,2′-[[pyrimidine]].<ref name="Yoshida745" />

The coordination chemistry of neptunium(V) has been extensively researched due to the presence of [[cation–cation interaction]]s in the solid state, which had been already known for [[actinyl]] ions.<ref name="Yoshida745" /> Some known such compounds include the neptunyl [[dimer (chemistry)|dimer]] {{chem|Na|4|(NpO|4|)|2|C|12|O|12}}·8H<sub>2</sub>O and neptunium [[glycolate]], both of which form green crystals.<ref name="Yoshida745" />

Neptunium(VI) compounds range from the simple oxalate {{chem|NpO|2|C|2|O|4}} (which is unstable, usually becoming Np(IV)) to such complicated compounds as the green {{chem|(NH|4|)|4|NpO|2|(CO|3|)|3}}.<ref name="Yoshida745" /> Extensive study has been performed on compounds of the form {{chem|M|4|AnO|2|(CO|3|)|3}}, where M represents a monovalent cation and An is either uranium, neptunium, or plutonium.<ref name="Yoshida745" />

Since 1967, when neptunium(VII) was discovered, some coordination compounds with neptunium in the +7 oxidation state have been prepared and studied. The first reported such compound was initially characterized as {{chem|Co(|NH|3|)|6|NpO|5}}·''n''H<sub>2</sub>O in 1968, but was suggested in 1973 to actually have the formula {{chem|[Co(|NH|3|)|6|][NpO|4|(OH)|2|]}}·2H<sub>2</sub>O based on the fact that Np(VII) occurs as {{chem|[NpO|4|(OH)|2|]|3-}} in aqueous solution.<ref name="Yoshida745" /> This compound forms dark green prismatic crystals with maximum edge length 0.15–0.4&nbsp;[[millimeter|mm]].<ref name="Yoshida745" />

====In aqueous solution====
Most neptunium [[coordination complex]]es known in solution involve the element in the +4, +5, and +6 oxidation states: only a few studies have been done on neptunium(III) and (VII) coordination complexes.<ref name="Yoshida77182">Yoshida et al., pp. 771–82.</ref> For the former, NpX<sup>2+</sup> and {{chem|NpX|2|+}} (X = [[chlorine|Cl]], [[bromine|Br]]) were obtained in 1966 in concentrated [[lithium chloride|LiCl]] and [[lithium bromide|LiBr]] solutions, respectively: for the latter, 1970 experiments discovered that the {{chem|NpO|2|3+}} ion could form [[sulfate]] complexes in acidic solutions, such as {{chem|NpO|2|SO|4|+}} and {{chem|NpO|2|(SO|4|)|2|-}}; these were found to have higher [[equilibrium constant|stability constants]] than the neptunyl ion ({{chem|NpO|2|2+}}).<ref name="Yoshida77182" /> A great many complexes for the other neptunium oxidation states are known: the inorganic ligands involved are the [[halide]]s, [[iodate]], [[azide]], [[nitride]], [[nitrate]], [[thiocyanate]], [[sulfate]], [[carbonate]], [[Chromate ion|chromate]], and [[phosphate]]. Many organic ligands are known to be able to be used in neptunium coordination complexes: they include [[acetate]], [[propionate]], [[glycolate]], [[lactic acid|lactate]], [[oxalate]], [[malonate]], [[phthalate]], [[mellitate]], and [[citrate]].<ref name="Yoshida77182" />

Analogously to its neighbours, uranium and plutonium, the order of the neptunium ions in terms of complex formation ability is Np<sup>4+</sup> > {{chem|NpO|2|2+}} ≥ Np<sup>3+</sup> > {{chem|NpO|2|+}}. (The relative order of the middle two neptunium ions depends on the [[ligand]]s and solvents used.)<ref name="Yoshida77182" /> The stability sequence for Np(IV), Np(V), and Np(VI) complexes with monovalent inorganic ligands is [[fluoride|F<sup>−</sup>]] > [[dihydrogen phosphate|{{chem|H|2|PO|4|-}}]] > [[thiocyanate|SCN<sup>−</sup>]] > [[nitrate|{{chem|NO|3|-}}]] > [[chloride|Cl<sup>−</sup>]] > [[perchlorate|{{chem|ClO|4|-}}]]; the order for divalent inorganic ligands is [[carbonate|{{chem|CO|3|2-}}]] > [[Monohydrogen phosphate|{{chem|H|PO|4|2-}}]] > [[sulfate|{{chem|SO|4|2-}}]]. These follow the strengths of the corresponding [[acid]]s. The divalent ligands are more strongly complexing than the monovalent ones.<ref name="Yoshida77182" /> {{chem|NpO|2|+}} can also form the complex ions [{{chem|NpO|2|+|M|3+}}] (M = [[aluminium|Al]], [[gallium|Ga]], [[scandium|Sc]], [[indium|In]], [[iron|Fe]], [[chromium|Cr]], [[rhodium|Rh]]) in [[perchloric acid]] solution: the strength of interaction between the two cations follows the order Fe > In > Sc > Ga > Al.<ref name="Yoshida77182" /> The neptunyl and uranyl ions can also form a complex together.<ref name="Yoshida77182" />

==Applications==

===Precursor in plutonium-238 production===
{{Main article|Plutonium-238}}
An important use of <sup>237</sup>Np is as a precursor in [[plutonium-238]] production, where it is irradiated with neutrons to form [[Plutonium-238|<sup>238</sup>Pu]], an [[alpha emitter]] for [[radioisotope thermal generator]]s for spacecraft and military applications. <sup>237</sup>Np will capture a neutron to form <sup>238</sup>Np and [[beta decay]] with a half-life of just over two days to <sup>238</sup>Pu.<ref>{{cite journal|doi = 10.1016/j.enconman.2007.10.028|pages = 393–401|title = Review of recent advances of radioisotope power systems|date = 2008|author = Lange, R|journal = Energy Conversion and Management|volume = 49|last2 = Carroll|first2 = W.|issue = 3| bibcode=2008ECM....49..393L |url = https://zenodo.org/record/1258917}}</ref>

:<chem>^{237}_{93}Np + ^{1}_{0}n -> ^{238}_{93}Np ->[\beta^-][2.117 \ \ce{d}] ^{238}_{94}Pu</chem>

<sup>238</sup>Pu also exists in sizable quantities in [[spent nuclear fuel]] but would have to be separated from other [[isotopes of plutonium]].<ref name="Yoshida703">Yoshida et al., pp. 702–3.</ref> Irradiating neptunium-237 with electron beams, provoking [[bremsstrahlung]], also produces quite pure samples of the isotope [[plutonium-236]], useful as a tracer to determine plutonium concentration in the environment.<ref name="Yoshida703" />

===Nuclear weapons===
Neptunium is [[fissionable]], and could theoretically be used as fuel in a [[fast-neutron reactor]] or a [[nuclear weapon]], with a [[critical mass]] of around 60 kilograms.<ref name="rsc" /> In 1992, the [[U.S. Department of Energy]] declassified the statement that neptunium-237 "can be used for a nuclear explosive device".<ref name="RDD-7">[https://fas.org/sgp/othergov/doe/rdd-7.html "Restricted Data Declassification Decisions from 1946 until Present"], accessed Sept 23, 2006.</ref> It is not believed that an actual weapon has ever been constructed using neptunium. As of 2009, the world production of neptunium-237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.<ref name="Nevada" />

In September 2002, researchers at the [[Los Alamos National Laboratory]] briefly produced the first known nuclear [[critical mass]] using a significant fraction of neptunium, in combination with shells of [[enriched uranium]] ([[uranium-235]]), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties,"<ref name="criticality">{{cite web
|url=http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/2.14_107.pdf
|title=Criticality of a <sup>237</sup>Np Sphere
|first1=Rene G.
|last1=Sanchez
|first2=David J.
|last2=Loaiza
|first3=Robert H.
|last3=Kimpland
|first4=David K.
|last4=Hayes
|first5=Charlene C.
|last5=Cappiello
|first6=William L.
|last6=Myers
|first7=Peter J.
|last7=Jaegers
|first8=Steven D.
|last8=Clement
|first9=Kenneth B.
|last9=Butterfield
|publisher=Japanese Atomic Energy Agency
|access-date=2014-08-06
|archive-date=2014-05-12
|archive-url=https://web.archive.org/web/20140512214219/http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/2.14_107.pdf
|url-status=dead
}}</ref> showing that it "is about as good a bomb material as [uranium-235]."<ref name="Weiss" /> The United States Federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in [[Nevada]].<ref name="Nevada" />

===Physics===
<sup>237</sup>Np is used in devices for detecting high-energy (MeV) neutrons.<ref>{{cite book| page =236| title =Experimental techniques in nuclear physics| publisher =Walter de Gruyter| date = 1997| isbn =978-3-11-014467-3| author1 =Dorin N. Poenaru| author2 =Walter Greiner| author-link1 =Dorin N. Poenaru| author-link2 =Walter Greiner}}</ref>
<!--{{cite journal|doi = 10.1016/0375-9474(72)90778-6|title = The energy dependence of the fissionability of neptunium isotopes and the level density of highly deformed nuclei|year = 1972|author = Bishop, C|journal = Nuclear Physics A|volume = 198|pages = 161–187 |bibcode = 1972NuPhA.198..161B|last2 = Halpern|first2 = I.|last3 = Shaw|first3 = R. W.|last4 = Vandenbosch|first4 = R. }} {{cite book|url = https://books.google.com/books?id=V1cwK1ehoYcC&pg=PT40|isbn = 978-3-527-32065-3|author = Hans-Jürgen Quadbeck-Seeger ; translated by José Oliveira.|year = 2007|publisher = Wiley-VCH|location = Weinheim|title = World of the elements : elements of the world}} -->

==Role in nuclear waste==
Neptunium accumulates in commercial household ionization-chamber [[smoke detector]]s from decay of the (typically) 0.2 [[microgram]] of americium-241 initially present as a source of [[ionizing radiation]]. With a half-life of 432 years, the americium-241 in an [[smoke detector#Design|ionization smoke detector]] includes about 3% neptunium after 20 years, and about 15% after 100 years.

Under [[Redox|oxidizing]] conditions, neptunium-237 is the most mobile [[actinide]] in the [[deep geological repository]] environment of the [[Yucca Mountain]] project in [[Nevada]].<ref>{{cite web| url = https://fas.org/sgp/othergov/doe/lanl/pubs/00818052.pdf| title= Yucca Mountain| access-date = 2009-06-06}}</ref> This makes it and its predecessors such as [[americium-241]] candidates of interest for destruction by [[nuclear transmutation]].<ref>{{cite journal| doi =10.1016/S0029-5493(03)00034-7| title =Deep-Burn: making nuclear waste transmutation practical| date =2003| author =Rodriguez, C| journal =Nuclear Engineering and Design| volume =222| pages =299–317| issue =2–3| display-authors =4| last2 =Baxter| first2 =A.| last3 =McEachern| first3 =D.| last4 =Fikani| first4 =M.| last5 =Venneri| first5 =F.| bibcode =2003NuEnD.222..299R}}</ref> Due to its long half-life, neptunium will become the major contributor of the total [[radiotoxicity]] at Yucca Mountain in 10,000 years. As it is unclear what happens to the non-reprocessed [[spent fuel]] containment in that long time span, an extraction and transmutation of neptunium after spent fuel reprocessing could help to minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.<ref name="Nevada">{{cite web|url = http://newscenter.lbl.gov/feature-stories/2005/11/29/getting-the-neptunium-out-of-nuclear-waste/|date =2005-11-29| title = Getting the Neptunium out of Nuclear Waste|first = Lynn|last = Yarris|publisher = Berkeley laboratory, U.S. Department of Energy|access-date = 2014-07-26}}</ref><ref>{{cite web|url = http://www.pnl.gov/main/publications/external/technical_reports/PNNL-14307.pdf|title = Existing Evidence for the Fate of Neptunium in the Yucca Mountain Repository|author = J. I. Friese|display-authors = 4|author2 = E. C. Buck|author3 = B. K. McNamara|author4 = B. D. Hanson|author5 = S. C. Marschman|date = 2003-01-06|publisher = Pacific northwest national laboratory, U.S. Department of Energy|access-date = 2014-07-26}}</ref><!--{{cite journal | doi = 10.1002/anie.200501281 | volume=44 | title=Optical Absorption and Structure of a Highly Symmetrical Neptunium(V) Diamide Complex | year=2005 | journal=Angewandte Chemie International Edition | pages=6200–6203 | last1 = Tian | first1 = Guoxin}}-->
<!--annual production 4.6 tonnes https://books.google.com/books?id=oFtPmEPqjCgC&pg=PA79-->

==Biological role and precautions==
Neptunium does not have a biological role, as it has a short half-life and occurs only in small traces naturally. Animal tests show it to be absorbed poorly (~1%) via the [[digestive tract]].<ref>{{cite web |title=Metabolism and gastrointestinal absorption of neptunium and protactinium in adult baboons |date=January 1985 |osti=5641940 |url=https://www.osti.gov/biblio/5641940 |access-date=24 July 2024 |last1=Ralston |first1=L. G. |last2=Cohen |first2=N. |last3=Bhattacharyya |first3=M. H. |last4=Larsen |first4=R. P. |last5=Ayres |first5=L. |last6=Oldham |first6=R. D. |last7=Moretti |first7=E. S. }}</ref> When injected it concentrates in the bones, from which it is slowly released.<ref name="emsley345347" />

Finely divided neptunium metal presents a fire hazard because neptunium is [[pyrophoricity|pyrophoric]]; small grains will ignite spontaneously in air at room temperature.<ref name="Yoshida724" />

==References==
{{reflist|30em}}

==Bibliography==
*{{cite book |last=Atwood |first=David A. |title=Radionuclides in the Environment |date=2013 |publisher=John Wiley and Sons| url=https://books.google.com/books?id=R5ATOuAFRlYC|isbn=9781118632697 }}
*{{cite book |last=Emsley |first=John |title=Nature's Building Blocks: An A-Z Guide to the Elements |date=2011 |publisher=Oxford University Press, USA |location=New York |isbn=978-0-199-60563-7}}
*{{cite book |last=Hoffman |first=Klaus |title=Otto Hahn: Achievement and Responsibility |date=2001 |publisher=Springer |isbn=978-0-387-95057-0 |bibcode=2002ohar.book.....H |url-access=registration |url=https://archive.org/details/ottohahnachievem0000hoff }}
*{{cite book |last=Lemire |first=Robert J.|title=Chemical Thermodynamics of Neptunium and Plutonium |date=2001| publisher=Elsevier| location=Amsterdam |isbn=978-0-444-50379-4 |url=https://books.google.com/books?id=ApDIq9BatBMC}}
*{{cite book |last=Rhodes |first=Richard |title=The Making of the Atomic Bomb |edition=25th Anniversary |date=2012 |publisher=Simon & Schuster |location=New York |isbn=978-1-451-67761-4}}
*{{cite book |last1=Yoshida |first1 = Zenko|first2 = Stephen G.|last2 = Johnson|first3 = Takaumi|last3 = Kimura|first4 = John R.|last4=Krsul|ref=Yoshida et al.|contribution = Neptunium|title = The Chemistry of the Actinide and Transactinide Elements|editor1-first = Lester R.|editor1-last = Morss|editor2-first = Norman M.|editor2-last = Edelstein|editor3-first = Jean|editor3-last = Fuger|edition = 3rd|date = 2006|volume = 3|publisher = Springer|location = Dordrecht, the Netherlands|pages = 699–812|url = http://radchem.nevada.edu/classes/rdch710/files/neptunium.pdf|doi = 10.1007/1-4020-3598-5_6|archive-url=https://web.archive.org/web/20180117190715/http://radchem.nevada.edu/classes/rdch710/files/neptunium.pdf|archive-date=January 17, 2018|isbn = 978-1-4020-3555-5}}

==Literature==
* ''Guide to the Elements – Revised Edition'', Albert Stwertka, (Oxford University Press; 1998) {{ISBN|0-19-508083-1}}
* Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): ''The Chemistry of the Actinide and Transactinide Elements'', Springer-Verlag, Dordrecht 2006, {{ISBN|1-4020-3555-1}}.
* {{cite journal|author=Ida Noddack|author-link=Ida Noddack|date=1934|pages=653–655|title=Über das Element 93|volume=47|journal=Zeitschrift für Angewandte Chemie|url=http://www.chemteam.info/Chem-History/Noddack-1934.html|doi=10.1002/ange.19340473707|issue=37|bibcode=1934AngCh..47..653N}}
* Eric Scerri, A Very Short Introduction to the Periodic Table, Oxford University Press, Oxford, 2011, {{ISBN|978-0-19-958249-5}}.

==External links==
{{Commons category|Neptunium}}
{{Wiktionary|Neptunium}}
* [http://www.periodicvideos.com/videos/093.htm Neptunium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://www.eurekalert.org/features/doe/2001-08/danl-lbw060502.php Lab builds world's first neptunium sphere] {{Webarchive|url=https://web.archive.org/web/20060925064716/http://www.eurekalert.org/features/doe/2001-08/danl-lbw060502.php |date=2006-09-25 }}, [[U.S. Department of Energy]] Research News
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+neptunium,+radioactive NLM Hazardous Substances Databank – Neptunium, Radioactive]
* [http://www.ead.anl.gov/pub/doc/neptunium.pdf Neptunium: Human Health Fact Sheet] {{Webarchive|url=https://web.archive.org/web/20040206045605/http://www.ead.anl.gov/pub/doc/neptunium.pdf |date=2004-02-06 }}
* [https://pubsapp.acs.org/cen/80th/neptunium.html? C&EN: It's Elemental: The Periodic Table – Neptunium]
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[[Category:Neptunium| ]]
[[Category:Chemical elements]]
[[Category:Actinides]]
[[Category:Synthetic elements]]
[[Category:Nuclear materials]]
[[Category:Pyrophoric materials]]