Editing Neptunium (section)

===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>