Editing Group 12 element (section)

===Classification===
The elements in group 12 are usually considered to be [[d-block]] elements, but not [[transition element]]s as the d-shell is full. Some authors classify these elements as [[main-group element]]s because the [[valence electron]]s are in ns<sup>2</sup> orbitals. Nevertheless, they share many characteristics with the neighboring [[group 11 element]]s on the periodic table, which are almost universally considered to be transition elements. For example, zinc shares many characteristics with the neighboring transition metal, copper. Zinc complexes merit inclusion in the [[Irving-Williams series]] as zinc forms many complexes with the same [[stoichiometry]] as complexes of copper(II), albeit with smaller [[stability constants of complexes|stability constants]].<ref>{{cite journal |last1=Al-Niaimi |first1=N. S. |last2=Hamid|first2=H. A. |year=1976 |title=Stabilities of nickel(II), copper(II), zinc(II) and dioxouranium(II) complexes of some β-diketones  |journal= Journal of Inorganic and Nuclear Chemistry |volume=3 |issue=5|doi=10.1016/0022-1902(77)80167-X |pages=849–852 }}</ref> There is little similarity between cadmium and silver as compounds of silver(II) are rare and those that do exist are very strong oxidizing agents. Likewise the common oxidation state for gold is +3, which precludes there being much common chemistry between mercury and gold, though there are similarities between mercury(I) and gold(I) such as the formation of linear dicyano complexes, [M(CN)<sub>2</sub>]<sup>−</sup>. According to [[IUPAC]]'s definition of transition metal as ''an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell'',<ref>{{GoldBookRef |title=transition element |file=T06456 }}</ref> zinc and cadmium are not transition metals, while mercury is. This is because only mercury is known to have a compound where its oxidation state is higher than +2, in [[mercury(IV) fluoride]] (though its existence is disputed, as later experiments trying to confirm its synthesis could not find evidence of HgF<sub>4</sub>).<ref>[http://www.speciation.net/News/Elusive-HgIV-species-has-been-synthesized-under-cryogenic-conditions-;~/2007/10/12/3303.html Elusive Hg(IV) species has been synthesized under cryogenic conditions]</ref><ref>{{cite journal|doi=10.1002/ange.200703710|title=Mercury is a Transition Metal: The First Experimental Evidence for HgF4|year=2007|last1=Wang|first1=Xuefang|last2=Andrews|first2=Lester|last3=Riedel|first3=Sebastian|last4=Kaupp|first4=Martin|journal=Angewandte Chemie|volume=119|issue=44|pages=8523–8527|bibcode=2007AngCh.119.8523W }}</ref> However, this classification is based on one highly atypical compound seen at non-equilibrium conditions and is at odds to mercury's more typical chemistry, and Jensen has suggested that it would be better to regard mercury as not being a transition metal.<ref name=Jensen/>