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{{for|the hacker group sometimes called NOBELIUM|Cozy Bear}}
{{infobox nobelium}}
'''Nobelium''' is a [[synthetic element|synthetic chemical element]]; it has [[Chemical symbol|symbol]] '''No''' and [[atomic number]] 102. It is named after [[Alfred Nobel]], the inventor of [[dynamite]] and benefactor of science. A [[radioactive]] [[metal]], it is the tenth [[transuranium element]], the second transfermium, and is the fourteenth member of the [[actinide series]]. Like all elements with atomic number over 100, nobelium can only be produced in [[particle accelerator]]s by bombarding lighter elements with charged particles. A total of twelve [[isotopes of nobelium|nobelium isotopes]] are known to exist; the most stable is <sup>259</sup>No with a [[half-life]] of 58 minutes, but the shorter-lived <sup>255</sup>No (half-life 3.1 minutes) is most commonly used in chemistry because it can be produced on a larger scale.

Chemistry experiments have confirmed that nobelium behaves as a heavier [[Homologous series|homolog]] to [[ytterbium]] in the periodic table. The chemical properties of nobelium are not completely known: they are mostly only known in [[aqueous solution]]. Before nobelium's discovery, it was predicted that it would show a stable +2 [[oxidation state]] as well as the +3 state characteristic of the other [[actinides]]; these predictions were later confirmed, as the +2 state is much more stable than the +3 state in [[aqueous solution]] and it is difficult to keep nobelium in the +3 state.

In the 1950s and 1960s, many claims of the discovery of nobelium were made from laboratories in [[Sweden]], the [[Soviet Union]], and the [[United States]]. Although the Swedish scientists soon retracted their claims, the priority of the discovery and therefore the [[Element naming controversy|naming of the element was disputed]] between Soviet and American scientists. It was not until 1992 that the [[International Union of Pure and Applied Chemistry]] (IUPAC) credited the Soviet team with the discovery. Even so, nobelium, the Swedish proposal, was retained as the name of the element due to its long-standing use in the literature.

==Introduction==
{{Excerpt|Superheavy element|Introduction|subsections=yes}}

==Discovery==
[[File:AlfredNobel2.jpg|thumb|right|The element was named after [[Alfred Nobel]]]]
The discovery of element 102 was a complicated process and was claimed by groups from [[Sweden]], the [[United States]], and the [[Soviet Union]]. The first complete and incontrovertible report of its [[discovery of the chemical elements|detection]] only came in 1966 from the [[JINR|Joint Institute of Nuclear Research]] at [[Dubna]] (then in the Soviet Union).<ref name="93TWG">{{Cite journal |doi=10.1351/pac199365081757 |title=Discovery of the transfermium elements. Part II: Introduction to discovery profiles. Part III: Discovery profiles of the transfermium elements |year=1993 |last1=Barber |first1=Robert C. |journal=Pure and Applied Chemistry |volume=65 |page=1757 |last2=Greenwood |first2=Norman N. |last3=Hrynkiewicz |first3=Andrzej Z. |last4=Jeannin |first4=Yves P. |last5=Lefort |first5=Marc |last6=Sakai |first6=Mitsuo |last7=Úlehla |first7=Ivan M. |last8=Wapstra |first8=Aaldert Hendrik |last9=Wilkinson |first9=Denys H. |s2cid=195819585 |issue=8|doi-access=free }} (Note: for Part I see Pure and Applied Chemistry, vol. 63, no. 6, pp. 879–886, 1991)</ref>

The first announcement of the discovery of element 102 was announced by physicists at the Nobel Institute for Physics in Sweden in 1957. The team reported that they had bombarded a [[curium]] target with [[carbon-13]] ions for twenty-five hours in half-hour intervals. Between bombardments, [[ion-exchange]] chemistry was performed on the target. Twelve out of the fifty bombardments contained samples emitting (8.5&nbsp;±&nbsp;0.1)&nbsp;[[MeV]] [[alpha particle]]s, which were in drops which eluted earlier than [[fermium]] (atomic number ''Z''&nbsp;=&nbsp;100) and [[californium]] (''Z''&nbsp;=&nbsp;98). The [[half-life]] reported was 10&nbsp;minutes and was assigned to either <sup>251</sup>102 or <sup>253</sup>102, although the possibility that the alpha particles observed were from a presumably short-lived [[mendelevium]] (''Z''&nbsp;=&nbsp;101) isotope created from the electron capture of element 102 was not excluded.<ref name="93TWG" /> The team proposed the name ''nobelium'' (No) for the new element,<ref name="Silva16367" /><ref>{{cite journal |last1=Fields |first1=Peter R. |last2=Friedman |first2=Arnold M. |last3=Milsted |first3=John |last4=Atterling |first4=Hugo |last5=Forsling |first5=Wilhelm |last6=Holm |first6=Lennart W. |last7=Åström |first7=Björn |date=1 September 1957 |title=Production of the New Element 102 |url=https://archive.org/details/sim_physical-review_1957-09-01_107_5/page/1460 |journal=Physical Review |volume=107 |issue=5 |pages=1460–1462 |doi=10.1103/PhysRev.107.1460 |bibcode=1957PhRv..107.1460F }}</ref> which was immediately approved by IUPAC,<ref name="Emsley2011" /> a decision which the Dubna group characterized in 1968 as being hasty.<ref name="TWGresponse">{{cite journal |doi=10.1351/pac199365081815 |title=Responses on 'Discovery of the transfermium elements' by Lawrence Berkeley Laboratory, California; Joint Institute for Nuclear Research, Dubna; and Gesellschaft fur Schwerionenforschung, Darmstadt followed by reply to responses by the Transfermium Working Group |year=1993 |last1=Ghiorso |first1=Albert |last2=Seaborg |first2=Glenn T. |last3=Oganessian |first3=Yuri Ts. |last4=Zvara |first4=Ivo |last5=Armbruster |first5=Peter |last6=Hessberger |first6=F. P. |last7=Hofmann |first7=Sigurd |last8=Leino |first8=Matti E. |last9=Münzenberg |first9=Gottfried |last10=Reisdorf |first10=Willibrord |last11=Schmidt |first11=Karl-Heinz |journal=Pure and Applied Chemistry |volume=65 |issue=8 |pages=1815–1824 |doi-access=free }}</ref>

In 1958, scientists at the [[Lawrence Berkeley National Laboratory]] repeated the experiment. The Berkeley team, consisting of [[Albert Ghiorso]], [[Glenn T. Seaborg]], [[John R. Walton]] and [[Torbjørn Sikkeland]], used the new heavy-[[ion]] [[linear accelerator]] (HILAC) to bombard a curium target (95% <sup>244</sup>Cm and 5% <sup>246</sup>Cm) with <sup>13</sup>C and <sup>12</sup>C ions. They were unable to confirm the 8.5&nbsp;MeV activity claimed by the Swedes but were instead able to detect decays from fermium-250, supposedly the daughter of <sup>254</sup>102 (produced from the curium-246), which had an apparent [[half-life]] of ~3&nbsp;s. Probably this assignment was also wrong, as later 1963 Dubna work showed that the half-life of <sup>254</sup>No is significantly longer (about 50&nbsp;s). It is more likely that the observed alpha decays did not come from element 102, but rather from <sup>250m</sup>Fm.<ref name="93TWG" />

In 1959, the Swedish team attempted to explain the Berkeley team's inability to detect element 102 in 1958, maintaining that they did discover it. However, later work has shown that no nobelium isotopes lighter than <sup>259</sup>No (no heavier isotopes could have been produced in the Swedish experiments) with a half-life over 3&nbsp;minutes exist, and that the Swedish team's results are most likely from [[thorium]]-225, which has a half-life of 8&nbsp;minutes and quickly undergoes triple alpha decay to [[polonium]]-213, which has a decay energy of 8.53612&nbsp;MeV. This hypothesis is lent weight by the fact that thorium-225 can easily be produced in the reaction used and would not be separated out by the chemical methods used. Later work on nobelium also showed that the divalent state is more stable than the trivalent one and hence that the samples emitting the alpha particles could not have contained nobelium, as the divalent nobelium would not have eluted with the other trivalent actinides.<ref name="93TWG" /> Thus, the Swedish team later retracted their claim and associated the activity to background effects.<ref name="Emsley2011">{{cite book |first=John |last=Emsley |title=Nature's Building Blocks: An A-Z Guide to the Elements |url=https://books.google.com/books?id=4BAg769RfKoC&pg=PA368 |date=2011 |publisher=Oxford University Press |isbn=978-0-19-960563-7 |pages=368–9 }}</ref>

In 1959, the team continued their studies and claimed that they were able to produce an isotope that decayed predominantly by emission of an 8.3&nbsp;MeV alpha particle, with a [[half-life]] of 3 s with an associated 30% [[spontaneous fission]] branch. The activity was initially assigned to <sup>254</sup>102 but later changed to <sup>252</sup>102. However, they also noted that it was not certain that element 102 had been produced due to difficult conditions.<ref name="93TWG" /> The Berkeley team decided to adopt the proposed name of the Swedish team, "nobelium", for the element.<ref name="Emsley2011" />

:{{nuclide|curium|244}} + {{nuclide|carbon|12}} → {{nuclide|nobelium|256}}{{su|p=*}} → {{nuclide|nobelium|252}} + 4 {{su|b=0|p=1}}{{SubatomicParticle|neutron}}

Meanwhile, in Dubna, experiments were carried out in 1958 and 1960 aiming to synthesize element 102 as well. The first 1958 experiment bombarded [[plutonium-239]] and [[plutonium-241|-241]] with [[oxygen-16]] ions. Some alpha decays with energies just over 8.5&nbsp;MeV were observed, and they were assigned to <sup>251,252,253</sup>102, although the team wrote that formation of isotopes from [[lead]] or [[bismuth]] impurities (which would not produce nobelium) could not be ruled out. While later 1958 experiments noted that new isotopes could be produced from [[mercury (element)|mercury]], [[thallium]], lead, or bismuth impurities, the scientists still stood by their conclusion that element 102 could be produced from this reaction, mentioning a half-life of under 30&nbsp;seconds and a decay energy of (8.8&nbsp;±&nbsp;0.5)&nbsp;MeV. Later 1960 experiments proved that these were background effects. 1967 experiments also lowered the decay energy to (8.6&nbsp;±&nbsp;0.4)&nbsp;MeV, but both values are too high to possibly match those of <sup>253</sup>No or <sup>254</sup>No.<ref name="93TWG" /> The Dubna team later stated in 1970 and again in 1987 that these results were not conclusive.<ref name="93TWG" />

In 1961, Berkeley scientists claimed the discovery of [[lawrencium|element 103]] in the reaction of californium with [[boron]] and carbon ions. They claimed the production of the isotope <sup>257</sup>103, and also claimed to have synthesized an alpha decaying isotope of element 102 that had a half-life of 15&nbsp;s and alpha decay energy 8.2&nbsp;MeV. They assigned this to <sup>255</sup>102 without giving a reason for the assignment. The values do not agree with those now known for <sup>255</sup>No, although they do agree with those now known for <sup>257</sup>No, and while this isotope probably played a part in this experiment, its discovery was inconclusive.<ref name="93TWG" />

Work on element 102 also continued in Dubna, and in 1964, experiments were carried out there to detect alpha-decay daughters of element 102 isotopes by synthesizing element 102 from the reaction of a [[uranium-238]] target with [[neon]] ions. The products were carried along a [[silver]] catcher foil and purified chemically, and the isotopes <sup>250</sup>Fm and <sup>252</sup>Fm were detected. The yield of <sup>252</sup>Fm was interpreted as evidence that its parent <sup>256</sup>102 was also synthesized: as it was noted that <sup>252</sup>Fm could also be produced directly in this reaction by the simultaneous emission of an alpha particle with the excess neutrons, steps were taken to ensure that <sup>252</sup>Fm could not go directly to the catcher foil. The half-life detected for <sup>256</sup>102 was 8&nbsp;s, which is much higher than the more modern 1967 value of (3.2&nbsp;±&nbsp;0.2)&nbsp;s.<ref name="93TWG" /> Further experiments were conducted in 1966 for <sup>254</sup>102, using the reactions <sup>243</sup>[[americium|Am]](<sup>15</sup>[[nitrogen|N]],4n)<sup>254</sup>102 and <sup>238</sup>U(<sup>22</sup>Ne,6n)<sup>254</sup>102, finding a half-life of (50&nbsp;±&nbsp;10)&nbsp;s: at that time the discrepancy between this value and the earlier Berkeley value was not understood, although later work proved that the formation of the isomer <sup>250m</sup>Fm was less likely in the Dubna experiments than at the Berkeley ones. In hindsight, the Dubna results on <sup>254</sup>102 were probably correct and can be now considered a conclusive detection of element 102.<ref name="93TWG" />

One more very convincing experiment from Dubna was published in 1966 (though it was submitted in 1965), again using the same two reactions, which concluded that <sup>254</sup>102 indeed had a half-life much longer than the 3&nbsp;seconds claimed by Berkeley.<ref name="93TWG" /> Later work in 1967 at Berkeley and 1971 at the [[Oak Ridge National Laboratory]] fully confirmed the discovery of element 102 and clarified earlier observations.<ref name="Emsley2011" /> In December 1966, the Berkeley group repeated the Dubna experiments and fully confirmed them, and used this data to finally assign correctly the isotopes they had previously synthesized but could not yet identify at the time. Thus they claimed to have discovered nobelium in 1958 to 1961.<ref name="Emsley2011" />

:{{nuclide|uranium|238}} + {{nuclide|neon|22}} → {{nuclide|nobelium|260}}{{su|p=*}} → {{nuclide|nobelium|254}} + 6 {{su|b=0|p=1}}{{SubatomicParticle|neutron}}

[[File:Frederic and Irene Joliot-Curie.jpg|thumb|right|[[Frédéric Joliot]] and [[Irène Joliot-Curie]]]]
In 1969, the Dubna team carried out chemical experiments on element 102 and concluded that it behaved as the heavier homologue of [[ytterbium]]. The Russian scientists proposed the name ''joliotium'' (Jo) for the new element after [[Irène Joliot-Curie]], who had recently died, creating an [[Transfermium Wars|element naming controversy]] that would not be resolved for several decades, with each group using its own proposed names.<ref name="Emsley2011" /><ref>{{cite journal |last1=Karpenko |first1=V. |date=1980 |title=The Discovery of Supposed New Elements: Two Centuries of Errors |journal=Ambix |volume=27 |issue=2 |pages=77–102 |doi=10.1179/amb.1980.27.2.77}}</ref>

In 1992, the [[IUPAC]]-[[IUPAP]] Transfermium Working Group (TWG) reassessed the claims of discovery and concluded that only the Dubna work from 1966 correctly detected and assigned decays to nuclei with atomic number 102 at the time. The Dubna team are therefore officially recognised as the discoverers of nobelium, although it is possible that it was detected at Berkeley in 1959.<ref name="93TWG" /> This decision was criticized by Berkeley the following year, calling the reopening of the cases of elements 101 to 103 a "futile waste of time", while Dubna agreed with IUPAC's decision.<ref name="TWGresponse" />

In 1994, as part of an attempted resolution to the element naming controversy, IUPAC ratified names for elements 101–109. For element 102, it ratified the name ''nobelium'' (No) on the basis that it had become entrenched in the literature over the course of 30 years and that [[Alfred Nobel]] should be commemorated in this fashion.<ref name="IUPAC1997">{{cite journal |title=Names and symbols of transfermium elements |journal=Pure and Applied Chemistry |volume=69 |issue=12 |pages=2471–2473 |date=1997 |url=http://pac.iupac.org/publications/pac/pdf/1997/pdf/6912x2471.pdf |doi=10.1351/pac199769122471 }}</ref> Because of outcry over the 1994 names, which mostly did not respect the choices of the discoverers, a comment period ensued, and in 1995 IUPAC named element 102 ''flerovium'' (Fl) as part of a new proposal, after either [[Georgy Flyorov]] or his eponymous [[Flerov Laboratory of Nuclear Reactions]].<ref name="Haire">{{cite book |last1=Hoffmann |first1=Darleane C. |last2=Lee |first2=Diana M. |last3=Pershina |first3=Valeria |date=2006 |chapter=Transactinides and the future elements |editor-last=Morss |editor-first=Lester R. |editor2-last=Edelstein |editor2-first=Norman M. |editor3-last=Fuger |editor3-first=Jean |title=The Chemistry of the Actinide and Transactinide Elements |url=https://archive.org/details/chemistryactinid00katz |url-access=limited |edition=3rd |publisher=[[Springer (publisher)|Springer]] |page=[https://archive.org/details/chemistryactinid00katz/page/n2011 1660] |isbn=978-1-4020-3555-5 }}</ref> This proposal was also not accepted, and in 1997 the name ''nobelium'' was restored.<ref name="IUPAC1997" /> Today the name ''flerovium'', with the same symbol, refers to [[flerovium|element 114]].<ref name="IUPAC-names-114-116">{{cite press release |date=30 May 2012 |title=Element 114 is Named Flerovium and Element 116 is Named Livermorium |url=http://www.iupac.org/news/news-detail/article/element-114-is-named-flerovium-and-element-116-is-named-livermorium.html |publisher=[[International Union of Pure and Applied Chemistry|IUPAC]] |archive-url=https://web.archive.org/web/20120602010328/http://www.iupac.org/news/news-detail/article/element-114-is-named-flerovium-and-element-116-is-named-livermorium.html |archive-date= 2 June 2012 }}</ref>

==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 }}</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 |url-access=subscription }}</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 |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 |url-access=subscription }}</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" />

==Preparation and purification==
The isotopes of nobelium are mostly produced by bombarding actinide targets ([[uranium]], [[plutonium]], [[curium]], [[californium]], or [[einsteinium]]), with the exception of nobelium-262, which is produced as the [[decay product|daughter]] of lawrencium-262.<ref name="Silva16378" /> The most commonly used isotope, <sup>255</sup>No, can be produced from bombarding [[curium]]-248 or californium-249 with carbon-12: the latter method is more common. Irradiating a 350&nbsp;[[microgram|μg]]&nbsp;cm<sup>−2</sup> target of californium-249 with three&nbsp;trillion 73&nbsp;[[electronvolt|MeV]] carbon-12 ions per second for ten minutes can produce around 1200 nobelium-255 atoms.<ref name="Silva16378" />

Once the nobelium-255 is produced, it can be separated out similarly as used to purify the neighboring actinide mendelevium. The recoil [[momentum]] of the produced nobelium-255 atoms is used to bring them physically far away from the target from which they are produced, bringing them onto a thin foil of metal (usually [[beryllium]], [[aluminium]], [[platinum]], or [[gold]]) just behind the target in a vacuum: this is usually combined by trapping the nobelium atoms in a gas atmosphere (frequently [[helium]]), and carrying them along with a gas jet from a small opening in the reaction chamber. Using a long [[capillary tube]], and including [[potassium chloride]] aerosols in the helium gas, the nobelium atoms can be transported over tens of [[meter]]s.<ref name="Silva16389">{{harvnb|Silva|2011|pp=1638–9}}</ref> The thin layer of nobelium collected on the foil can then be removed with dilute acid without completely dissolving the foil.<ref name="Silva16389" /> The nobelium can then be isolated by exploiting its tendency to form the divalent state, unlike the other trivalent actinides: under typically used [[elution]] conditions ([[bis-(2-ethylhexyl) phosphoric acid]] (HDEHP) as stationary organic phase and 0.05&nbsp;M&nbsp;[[hydrochloric acid]] as mobile aqueous phase, or using 3&nbsp;M&nbsp;hydrochloric acid as an eluant from [[cation-exchange]] resin columns), nobelium will pass through the column and elute while the other trivalent actinides remain on the column.<ref name="Silva16389" /> However, if a direct "catcher" gold foil is used, the process is complicated by the need to separate out the gold using [[anion-exchange]] [[chromatography]] before isolating the nobelium by elution from [[chromatography|chromatographic]] extraction columns using HDEHP.<ref name="Silva16389" />

== Notes ==

{{Notelist}}

==References==
{{Reflist|30em}}

==Bibliography==

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* {{cite book |doi=10.1007/978-94-007-0211-0_13 |title=The Chemistry of the Actinide and Transactinide Elements |url=https://archive.org/details/chemistryactinid00mors |url-access=limited |pages=[https://archive.org/details/chemistryactinid00mors/page/n1639 1621]–1651 |date=2011 |isbn=978-94-007-0210-3 |publisher=Springer |place=Netherlands |last=Silva |first=Robert J. |editor-last=Morss |editor-first=Lester R. |editor2-last=Edelstein |editor2-first=Norman M. |editor3-last=Fuger |editor3-first=Jean |chapter=Chapter 13. Fermium, Mendelevium, Nobelium, and Lawrencium }}
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==External links==
{{Commons|Nobelium}}
* [http://www.nndc.bnl.gov/chart/ Chart of Nuclides] {{Webarchive|url=https://web.archive.org/web/20181010070007/http://www.nndc.bnl.gov/chart/ |date=2018-10-10 }}. nndc.bnl.gov
* [http://periodic.lanl.gov/102.shtml Los Alamos National Laboratory – Nobelium]
* [http://www.periodicvideos.com/videos/102.htm Nobelium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

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[[Category:Nobelium| ]]
[[Category:Chemical elements]]
[[Category:Chemical elements with face-centered cubic structure]]
[[Category:Actinides]]
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[[Category:Alfred Nobel]]