Editing Nobelium (section)

==Characteristics==

===Physical===
[[File:Fblock fd promotion energy.png|thumb|upright=1.6|right|Energy required to promote an f electron to the d subshell for the f-block lanthanides and actinides. Above around 210&nbsp;kJ/mol, this energy is too high to be provided for by the greater [[crystal energy]] of the trivalent state and thus einsteinium, fermium, and mendelevium form divalent metals like the lanthanides [[europium]] and [[ytterbium]]. Nobelium is also expected to form a divalent metal, but this has not yet been confirmed.<ref>{{cite book |first=Richard G. |last=Haire |ref=Haire |contribution=Einsteinium |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=1577–1620 |url=http://radchem.nevada.edu/classes/rdch710/files/einsteinium.pdf |doi=10.1007/1-4020-3598-5_12 |isbn=978-1-4020-3555-5 |access-date=2014-08-15 |archive-date=2010-07-17 |archive-url=https://web.archive.org/web/20100717154427/http://radchem.nevada.edu/classes/rdch710/files/einsteinium.pdf |url-status=dead }}</ref>]]
In the [[periodic table]], nobelium is located to the right of the actinide [[mendelevium]], to the left of the actinide [[lawrencium]], and below the lanthanide [[ytterbium]]. Nobelium metal has not yet been prepared in bulk quantities, and bulk preparation is currently impossible.<ref name="Silva1639">{{harvnb|Silva|2011|p=1639}}</ref> Nevertheless, a number of predictions and some preliminary experimental results have been done regarding its properties.<ref name="Silva1639" />

The lanthanides and actinides, in the metallic state, can exist as either divalent (such as [[europium]] and [[ytterbium]]) or trivalent (most other lanthanides) metals. The former have f<sup>''n''</sup>s<sup>2</sup> configurations, whereas the latter have f<sup>''n''−1</sup>d<sup>1</sup>s<sup>2</sup> configurations. In 1975, Johansson and Rosengren examined the measured and predicted values for the [[cohesive energy|cohesive energies]] ([[enthalpy|enthalpies]] of crystallization) of the metallic [[lanthanide]]s and [[actinide]]s, both as divalent and trivalent metals.<ref name="Silva16268">{{harvnb|Silva|2011|pp=1626–8}}</ref><ref>{{cite journal |doi=10.1103/PhysRevB.11.2836 |title=Generalized phase diagram for the rare-earth elements: Calculations and correlations of bulk properties |date=1975 |last1=Johansson |first1=Börje |last2=Rosengren |first2=Anders |journal=Physical Review B |volume=11 |pages=2836–2857 |bibcode=1975PhRvB..11.2836J |issue=8 }}</ref> The conclusion was that the increased binding energy of the [Rn]5f<sup>13</sup>6d<sup>1</sup>7s<sup>2</sup> configuration over the [Rn]5f<sup>14</sup>7s<sup>2</sup> configuration for nobelium was not enough to compensate for the energy needed to promote one 5f electron to 6d, as is true also for the very late actinides: thus [[einsteinium]], [[fermium]], [[mendelevium]], and nobelium were expected to be divalent metals, although for nobelium this prediction has not yet been confirmed.<ref name="Silva16268" /> The increasing predominance of the divalent state well before the actinide series concludes is attributed to the [[relativistic quantum chemistry|relativistic]] stabilization of the 5f electrons, which increases with increasing atomic number: an effect of this is that nobelium is predominantly divalent instead of trivalent, unlike all the other lanthanides and actinides.<ref>{{cite book |doi=10.1021/bk-1980-0131.ch012 |title=Lanthanide and Actinide Chemistry and Spectroscopy |volume=131 |pages=[https://archive.org/details/lanthanideactini0000unse/page/239 239–263] |date=1980 |isbn=978-0-8412-0568-0 |last=Hulet |first=E. Kenneth |editor-last=Edelstein |editor-first=Norman M. |chapter=Chapter 12. Chemistry of the Heaviest Actinides: Fermium, Mendelevium, Nobelium, and Lawrencium |series=ACS Symposium Series |chapter-url-access=registration |chapter-url=https://archive.org/details/lanthanideactini0000unse |url=https://archive.org/details/lanthanideactini0000unse/page/239 }}</ref> In 1986, nobelium metal was estimated to have an [[enthalpy of sublimation]] between 126&nbsp;kJ/mol, a value close to the values for einsteinium, fermium, and mendelevium and supporting the theory that nobelium would form a divalent metal.<ref name="Silva1639" /> Like the other divalent late actinides (except the once again trivalent lawrencium), metallic nobelium should assume a [[face-centered cubic]] crystal structure.<ref name="density" /> Divalent nobelium metal should have a [[metallic radius]] of around 197&nbsp;[[picometer|pm]].<ref name="Silva1639" /> Nobelium's melting point has been predicted to be 800&nbsp;°C, the same value as that estimated for the neighboring element mendelevium.<ref>{{cite book |ref=Haynes |editor-last=Haynes |editor-first=William M. |date=2011 |title= CRC Handbook of Chemistry and Physics |edition=92nd |publisher=CRC Press |isbn=978-1-4398-5511-9 |pages=4.121–4.123 }}</ref> Its density is predicted to be around 9.9&nbsp;±&nbsp;0.4&nbsp;g/cm<sup>3</sup>.<ref name="density" />

===Chemical===
The chemistry of nobelium is incompletely characterized and is known only in aqueous solution, in which it can take on the +3 or +2 [[oxidation state]]s, the latter being more stable.<ref name="Silva16367">{{harvnb|Silva|2011|pp=1636–7}}</ref> It was largely expected before the discovery of nobelium that in solution, it would behave like the other actinides, with the trivalent state being predominant; however, Seaborg predicted in 1949 that the +2 state would also be relatively stable for nobelium, as the No<sup>2+</sup> ion would have the ground-state electron configuration [Rn]5f<sup>14</sup>, including the stable filled 5f<sup>14</sup> shell. It took nineteen years before this prediction was confirmed.<ref name="Silva163941">{{harvnb|Silva|2011|pp=1639–41}}</ref>

In 1967, experiments were conducted to compare nobelium's chemical behavior to that of [[terbium]], [[californium]], and [[fermium]]. All four elements were reacted with [[chlorine]] and the resulting chlorides were deposited along a tube, along which they were carried by a gas. It was found that the nobelium chloride produced was strongly [[adsorption|adsorbed]] on solid surfaces, proving that it was not very [[volatility (chemistry)|volatile]], like the chlorides of the other three investigated elements. However, both NoCl<sub>2</sub> and NoCl<sub>3</sub> were expected to exhibit nonvolatile behavior and hence this experiment was inconclusive as to what the preferred oxidation state of nobelium was.<ref name="Silva163941" /> Determination of nobelium's favoring of the +2 state had to wait until the next year, when [[cation-exchange chromatography]] and [[coprecipitation]] experiments were carried out on around fifty thousand <sup>255</sup>No atoms, finding that it behaved differently from the other actinides and more like the divalent [[alkaline earth metal]]s. This proved that in aqueous solution, nobelium is most stable in the divalent state when strong [[redox|oxidizers]] are absent.<ref name="Silva163941" /> Later experimentation in 1974 showed that nobelium eluted with the alkaline earth metals, between [[calcium|Ca]]<sup>2+</sup> and [[strontium|Sr]]<sup>2+</sup>.<ref name="Silva163941" /> Nobelium is the only known f-block element for which the +2 state is the most common and stable one in aqueous solution. This occurs because of the large energy gap between the 5f and 6d orbitals at the end of the actinide series.<ref>{{Greenwood&Earnshaw|p=1278}}</ref>

It is expected that the relativistic stabilization of the 7s subshell greatly destabilizes nobelium dihydride, NoH<sub>2</sub>, and relativistic stabilisation of the 7p<sub>1/2</sub> spinor over the 6d<sub>3/2</sub> spinor mean that excited states in nobelium atoms have 7s and 7p contribution instead of the expected 6d contribution. The long No–H distances in the NoH<sub>2</sub> molecule and the significant charge transfer lead to extreme ionicity with a [[molecular dipole moment|dipole moment]] of 5.94&nbsp;[[debye|D]] for this molecule. In this molecule, nobelium is expected to exhibit [[main-group element|main-group-like]] behavior, specifically acting like an [[alkaline earth metal]] with its ''n''s<sup>2</sup> valence shell configuration and core-like 5f orbitals.<ref>{{cite journal |last1=Balasubramanian |first1=Krishnan |date=4 December 2001 |title=Potential energy surfaces of Lawrencium and Nobelium dihydrides (LrH<sub>2</sub> and NoH<sub>2</sub>)… |journal=Journal of Chemical Physics |volume=116 |issue=9 |pages=3568–75 |doi=10.1063/1.1446029 |bibcode=2002JChPh.116.3568B }}</ref>

Nobelium's [[coordination complex|complexing]] ability with [[chloride]] ions is most similar to that of [[barium]], which complexes rather weakly.<ref name="Silva163941" /> Its complexing ability with [[citrate]], [[oxalate]], and [[acetate]] in an aqueous solution of 0.5&nbsp;M&nbsp;[[ammonium nitrate]] is between that of calcium and strontium, although it is somewhat closer to that of strontium.<ref name="Silva163941" />

The [[standard reduction potential]] of the ''E''°(No<sup>3+</sup>→No<sup>2+</sup>) couple was estimated in 1967 to be between +1.4&nbsp;and&nbsp;+1.5&nbsp;[[volt|V]];<ref name="Silva163941" /> it was later found in 2009 to be only about +0.75&nbsp;V.<ref>{{cite journal |last1=Toyoshima |first1=A. |last2=Kasamatsu |first2=Y. |first3=K. |last3=Tsukada |first4=M. |last4=Asai |first5=Y. |last5=Kitatsuji |first6=Y. |last6=Ishii |first7=H. |last7=Toume |first8=I. |last8=Nishinaka |first9=H. |last9=Haba |first10=K. |last10=Ooe |first11=W. |last11=Sato |first12=A. |last12=Shinohara |first13=K. |last13=Akiyama |first14=Y. |last14=Nagame |date=8 July 2009 |title=Oxidation of element 102, nobelium, with flow electrolytic column chromatography on an atom-at-a-time scale |journal=Journal of the American Chemical Society |volume=131 |issue=26 |pages=9180–1 |doi=10.1021/ja9030038 |pmid=19514720 |bibcode=2009JAChS.131.9180T |url=https://figshare.com/articles/Oxidation_of_Element_102_Nobelium_with_Flow_Electrolytic_Column_Chromatography_on_an_Atom_at_a_Time_Scale/2844817 }}</ref> The positive value shows that No<sup>2+</sup> is more stable than No<sup>3+</sup> and that No<sup>3+</sup> is a good oxidizing agent. While the quoted values for the ''E''°(No<sup>2+</sup>→No<sup>0</sup>) and ''E''°(No<sup>3+</sup>→No<sup>0</sup>) vary among sources, the accepted standard estimates are −2.61&nbsp;and&nbsp;−1.26&nbsp;V.<ref name="Silva163941" /> It has been predicted that the value for the ''E''°(No<sup>4+</sup>→No<sup>3+</sup>) couple would be +6.5&nbsp;V.<ref name="Silva163941" /> The [[Gibbs energy|Gibbs energies]] of formation for No<sup>3+</sup> and No<sup>2+</sup> are estimated to be −342&nbsp;and&nbsp;−480&nbsp;[[kilojoule per mole|kJ/mol]], respectively.<ref name="Silva163941" />

===Atomic===
A nobelium atom has 102 electrons. They are expected to be arranged in the configuration [Rn]5f<sup>14</sup>7s<sup>2</sup> (ground state [[term symbol]] <sup>1</sup>S<sub>0</sub>), although experimental verification of this electron configuration had not yet been made as of 2006. The sixteen electrons in the 5f and 7s subshells are [[valence electron]]s.<ref name="Silva1639" /> In forming compounds, three valence electrons may be lost, leaving behind a [Rn]5f<sup>13</sup> core: this conforms to the trend set by the other actinides with their [Rn]5f<sup>''n''</sup> electron configurations in the tripositive state. Nevertheless, it is more likely that only two valence electrons are lost, leaving behind a stable [Rn]5f<sup>14</sup> core with a filled 5f<sup>14</sup> shell. The first [[ionization potential]] of nobelium was measured to be at most (6.65&nbsp;±&nbsp;0.07)&nbsp;[[electronvolt|eV]] in 1974, based on the assumption that the 7s electrons would ionize before the 5f ones;<ref name="NIST">{{cite journal |first1=William C. |last1=Martin |first2=Lucy |last2=Hagan |first3=Joseph |last3=Reader |first4=Jack |last4=Sugar |s2cid=97945150 |date=1974 |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |journal=[[Journal of Physical and Chemical Reference Data]] |volume=3 |issue=3 |pages=771–9 |doi=10.1063/1.3253147 |bibcode=1974JPCRD...3..771M |url=https://pdfs.semanticscholar.org/9618/febdd51cee0e84ff7af88767be47cfcd4818.pdf |archive-url=https://web.archive.org/web/20200215124722/https://pdfs.semanticscholar.org/9618/febdd51cee0e84ff7af88767be47cfcd4818.pdf |url-status=dead |archive-date=2020-02-15 }}</ref> this value has not yet been refined further due to nobelium's scarcity and high radioactivity.<ref>Lide, David R. (editor), ''CRC Handbook of Chemistry and Physics, 84th Edition'', CRC Press, Boca Raton (FL), 2003, section 10, ''Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions''</ref> The ionic radius of [[coordination number|hexacoordinate]] and octacoordinate No<sup>3+</sup> had been preliminarily estimated in 1978 to be around 90 and 102&nbsp;pm respectively;<ref name="Silva163941" /> the ionic radius of No<sup>2+</sup> has been experimentally found to be 100&nbsp;pm to two [[significant figure]]s.<ref name="Silva1639" /> The [[enthalpy of hydration]] of No<sup>2+</sup> has been calculated as 1486&nbsp;kJ/mol.<ref name="Silva163941" />

===Isotopes===
{{Main|Isotopes of nobelium}}

Fourteen isotopes of nobelium are known, with [[mass number]]s 248–260 and 262; all are radioactive.{{NUBASE2020|ref}} Additionally, [[nuclear isomer]]s are known for mass numbers 250, 251, 253, and 254.<ref name="unc">{{Cite web | url=http://www.nucleonica.net/unc.aspx | title=Nucleonica :: Web driven nuclear science}}</ref><ref name="NUBASE2003" /> Of these, the longest-lived isotope is <sup>259</sup>No with a half-life of 58&nbsp;minutes, and the longest-lived isomer is <sup>251m</sup>No with a half-life of 1.7&nbsp;seconds.<ref name="unc" /><ref name="NUBASE2003">{{NUBASE 2003}}</ref> However, the still undiscovered isotope <sup>261</sup>No is predicted to have a still longer half-life of 3&nbsp;hours.{{NUBASE2020|ref}} Additionally, the shorter-lived <sup>255</sup>No (half-life 3.1&nbsp;minutes) is more often used in chemical experimentation because it can be produced in larger quantities from irradiation of [[californium-249]] with [[carbon-12]] ions.<ref name="Silva16378" /> After <sup>259</sup>No and <sup>255</sup>No, the next most stable nobelium isotopes are <sup>253</sup>No (half-life 1.62&nbsp;minutes), <sup>254</sup>No (51&nbsp;[[second]]s), <sup>257</sup>No (25&nbsp;seconds), <sup>256</sup>No (2.91&nbsp;seconds), and <sup>252</sup>No (2.57&nbsp;seconds).<ref name="Silva16378" /><ref name="unc" /><ref name="NUBASE2003" /> All of the remaining nobelium isotopes have half-lives that are less than a second, and the shortest-lived known nobelium isotope (<sup>248</sup>No) has a half-life of less than 2&nbsp;[[microsecond]]s.{{NUBASE2020|ref}} The isotope <sup>254</sup>No is especially interesting theoretically as it is in the middle of a series of [[prolate]] nuclei from <sup>231</sup>[[protactinium|Pa]] to <sup>279</sup>[[roentgenium|Rg]], and the formation of its nuclear isomers (of which two are known) is controlled by [[nuclear shell model|proton orbitals]] such as 2f<sub>5/2</sub> which come just above the spherical proton shell; it can be synthesized in the reaction of <sup>208</sup>Pb with <sup>48</sup>Ca.<ref name="Kratz">{{cite conference |last1=Kratz |first1=Jens Volker |date=5 September 2011 |title=The Impact of Superheavy Elements on the Chemical and Physical Sciences |url=http://tan11.jinr.ru/pdf/06_Sep/S_1/02_Kratz.pdf |conference=4th International Conference on the Chemistry and Physics of the Transactinide Elements |access-date=27 August 2013 }}</ref>

The half-lives of nobelium isotopes increase smoothly from <sup>250</sup>No to <sup>253</sup>No. However, a dip appears at <sup>254</sup>No, and beyond this the half-lives of [[even and odd atomic nuclei|even-even]] nobelium isotopes drop sharply as [[spontaneous fission]] becomes the dominant decay mode. For example, the half-life of <sup>256</sup>No is almost three seconds, but that of <sup>258</sup>No is only 1.2&nbsp;milliseconds.<ref name="Silva16378">{{harvnb|Silva|2011|pp=1637–8}}</ref><ref name="unc" /><ref name="NUBASE2003" /> This shows that at nobelium, the mutual repulsion of protons poses a limit to the [[island of stability|region of long-lived nuclei]] in the [[actinide]] series.<ref name="Nurmia">{{cite journal |first=Matti |last=Nurmia |date=2003 |title=Nobelium |journal=Chemical and Engineering News |url=http://pubs.acs.org/cen/80th/nobelium.html |volume=81 |issue=36 |page=178 |doi=10.1021/cen-v081n036.p178 }}</ref> The even-odd nobelium isotopes mostly continue to have longer half-lives as their mass numbers increase, with a dip in the trend at <sup>257</sup>No.<ref name="Silva16378" /><ref name="unc" /><ref name="NUBASE2003" />