Editing Nobelium (section)

===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" />