Group 12 element

A group 12 element is one of the elements in group 12 (IUPAC style) in the periodic table, consisting of zinc (Zn), cadmium (Cd) and mercury (Hg).[1][2][3] The inclusion of copernicium (Cn) in group 12 is supported by recent experiments on individual Cn atoms.[4]

All elements in this group are metals. The similarity of the metallic radii of cadmium and mercury is a knock-on effect of the lanthanide contraction. So, the trend in this group is unlike the trend in group 2, the alkaline earths, where metallic radius increases smoothly from top to bottom of the group. All three metals have relatively low melting and boiling points, which indicates that the metallic bond is relatively weak, with relatively little overlap between the valence band and the conduction band. Thus, zinc is close to the boundary between metallic and metalloid elements which is usually placed between gallium and germanium, though gallium participates in semi-conductors such as gallium arsenide.

Zinc is the most electropositive element in the group and zinc metal is a good reducing agent. The group oxidation state is +2 in which the ions have the rather stable d¹⁰ electronic configuration, with a full sub-shell. However, mercury can easily be reduced to the +1 oxidation state; usually, as in the ion Hg₂²⁺, two mercury(I) ions come together to form a metal-metal bond and a diamagnetic species. Cadmium can also form species such as[Cd₂Cl₆]⁴⁻ in which the metal's oxidation state is +1. Just as with mercury, the formation of a metal-metal bond results in a diamagnetic compound in which there are no unpaired electrons which would otherwise make the species very reactive. Zinc(I) is known only in the gas phase, in such compounds as linear Zn₂Cl₂, analogous to calomel.

All three metal ions form many tetrahedral species, such as MCl₄²⁻. When a divalent ion of these elements forms a tetrahedral complex, it obeys the octet rule. Both zinc and cadmium can also form octahedral complexes such as the aqua ions [M(H₂O)₆]²⁺ which are present in aqueous solutions of salts of these metals. Covalent character is achieved by using the 4d or 5d orbitals, respectively, forming sp³d² hybrid orbitals. Mercury, however, rarely exceeds a coordination number of four; when it does so the 5f orbitals must be involved. Coordination numbers of 2, 3, 5, 7 and 8 are also known.

The elements in group 12 are usually considered to be d-block elements, but not transition elements as the d-shell is full. Some authors classify these elements as main-group elements because the valence electrons are in ns2 orbitals. Nevertheless zinc shares many characteristics with the neighbouring transition metal, copper. For instance, 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. 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)₂]⁻.