Modern inorganic chemistry

434 THE ELEMENTS OF GROUPS IB AND IIB

Table 14.2 shows that all three elements have remarkably low

melting points and boiling points—an indication of the weak

metallic ^bonding, especially notable in mercury. The low heat of

atomisation of the latter element compensates to some extent its

higher ionisation energies, so that, in practice, all the elements of

this group can form cations M^2 + in aqueous solution or in hydrated

salts; anhydrous mercury(II) compounds are generally covalent.

CADMIUM

THE ELEMENT

Cadmium is usually found in zinc ores and is extracted from them
along with zinc (p. 416); it may be separated from the zinc by
distillation (cadmium is more volatile than zinc, Table 14.2) or by
electrolytic deposition.
Cadmium is a soft metal, which forms a protective coating in air,
and bums only on strong heating to give the brown oxide CdO. It
dissolves in acids with evolution of hydrogen :

Cd2+(aq) + 2e~ -> Cd(s): E^ = -0.40 V

It is used as a protective agent, particularly for iron, and is more
resistant to corrosion by sea water than, for example, zinc or nickel.
In its chemistry, cadmium exhibits exclusively the oxidation state
-f 2 in both ionic and covalent compounds. The hydroxide is soluble
in acids to give cadmium(II) salts, and slightly soluble in con-
centrated alkali where hydroxocadmiates are probably formed; it
is therefore slightly amphoteric. It is also soluble in ammonia to
give ammines, for example [Cd(NH 3 ) 4 ]2+. Of the halides, cadmium-
ill) chloride is soluble in water, but besides [Cd(H 2 O)J^2 + ions,
complex species [CdCl], [CdQ 3 ]~ and the undissociated chloride
[CdCl 2 ] exist in the solution, and addition of chloride ion increases
the concentrations of these chloro-complexes at the expense of
Cd2+(aq) ions.
Solid cadmium(II) iodide CdI 2 has a layer lattice' — a structure
intermediate between one containing Cd^2 and I~ ions and one
containing CdI 2 molecules — and this on vaporisation gives linear,
covalent I — Cd — I molecules. In solution, iodo-complexes exist, for
example

Cadmium(ll} sulphide, CdS, is a canary-yellow solid, precipitated by
addition of hydrogen sulphide (or sulphide ion) to an acid solution