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32-column periodic table, with bifurcated group 3

In this variant, group 3 bifurcates after Sc-Y into a La-Ac branch, and a Lu-Lr branch. This arrangeme

In this variant, group 3 bifurcates after Sc-Y into a La-Ac branch, and a Lu-Lr branch. This arrangement is consistent with the hypothesis that arguments in favour of either Sc-Y-La-Ac or Sc-Y-Lu-Lr based on chemical and physical data are inconclusive.[200] As noted, trends going down Sc-Y-La-Ac match trends in groups 1−2[201] whereas trends going down Sc-Y-Lu-Lr better match trends in groups 4−10.[185]

The bifurcation of group 3 is a throwback to the Mendeleev eight column-form in which seven of the main groups each have two subgroups. Tables featuring a bifurcated group 3 have been periodically proposed since that time.[n 18]

Groups included in the transition metals

The definition of a transition metal, as given by IUPAC in the Gold Book, is an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.[202] By this definition all of the elements in groups 3–11 are transition metals. The IUPAC definition therefore excludes group 12, comprising zinc,

The bifurcation of group 3 is a throwback to the Mendeleev eight column-form in which seven of the main groups each have two subgroups. Tables featuring a bifurcated group 3 have been periodically proposed since that time.[n 18]

The definition of a transition metal, as given by IUPAC in the Gold Book, is an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.[202] By this definition all of the elements in groups 3–11 are transition metals. The IUPAC definition therefore excludes group 12, comprising zinc, cadmium and mercury, from the transition metals category. However, the 2005 IUPAC nomenclature as codified in the Red Book gives both the group 3–11 and group 3–12 definitions of the transition metals as alternatives.

Some chemists treat the categories "d-block elements" and "tran

Some chemists treat the categories "d-block elements" and "transition metals" interchangeably, thereby including groups 3–12 among the transition metals. In this instance the group 12 elements are treated as a special case of transition metal in which the d electrons are not ordinarily given up for chemical bonding (they can sometimes contribute to the valence bonding orbitals even so, as in zinc fluoride).[203] The 2007 report of mercury(IV) fluoride (HgF4), a compound in which mercury would use its d electrons for bonding, has prompted some commentators to suggest that mercury can be regarded as a transition metal.[204] Other commentators, such as Jensen,[205] have argued that the formation of a compound like HgF4 can occur only under highly abnormal conditions; indeed, its existence is currently disputed. As such, mercury could not be regarded as a transition metal by any reasonable interpretation of the ordinary meaning of the term.[205]

Still other chemists further exclude the group 3 elements from the definition of a transition metal. They do so on the basis that the group 3 elements do not form any ions having a partially occupied d shell and do not therefore exhibit properties characteristic of transition metal chemistry.[206] In this case, only groups 4–11 are regarded as transition metals. This categorisation is however not one of the alternatives considered by IUPAC. Though the group 3 elements show few of the characteristic chemical properties of the transition metals, the same is true of the heavy members of groups 4 and 5, which also are mostly restricted to the group oxidation state in their chemistry. Moreover, the group 3 elements show characteristic physical properties of transition metals (on account of the presence in each atom of a single d electron).[85]