Modern inorganic chemistry

THE TRANSITION ELEMENTS 417

calamine, to give the oxide ZnO, which is then reduced to the metal
by heating with coke, or (b) by dissolving out the zinc content of the
ore with sulphuric acid, to give a solution of zinc(II) sulphate,
ZnSO^, which is electrolysed with an aluminium cathode on which
the zinc metal is deposited.
The data provided at the beginning of this chapter show that zinc
has a melting point and boiling point much lower than the preceding
transition metals. This allows zinc to be melted or distilled without
difficulty, and distillation may be used to purify zinc from less
volatile metals. The low boiling point is an indication of weak
metallic bonding, which in turn indicates that the filled 3d electron
levels are not extensively involved in forming zinc-zinc bonds in
the metal. Moreover, zinc in its chemical behaviour shows few
characteristics of a transition element; it exhibits only one oxidaton
state, + 2, in either ionic or covalent compounds, indicating the
involvement only of the two outer, 4s electrons. Its compounds are
commonly colourless, but it does show a somewhat greater tendency
to form complexes than the analogous elements (Ca, Sr, Ba) of
Group II.
The metal is not attacked by dry air at ordinary temperature; in
moist air it tarnishes, forming a basic carbonate which acts as a
coating preventing further corrosion. When heated in air, it burns
with a greenish-blue flame giving a fibrous deposit of zinc oxide.
This was the 'philosopher's wool' of the alchemists. Zinc combines
directly with chlorine and sulphur but not with nitrogen (cf. mag-
nesium), although the compound zinc nitride, Zn 3 N 2 , can be
obtained by passing ammonia over red-hot zinc. The metal does not
react with water but steam attacks it at red heat (cf. magnesium):

H 2 O + Zn -» ZnO 4- H 2

Despite its electrode potential (p. 98), very pure zinc has little or
no reaction with dilute acids. If impurities are present, local electro-
chemical 'cells' are set up (cf. the rusting of iron, p, 398) and the zinc
reacts readily evolving hydrogen. Amalgamation of zinc with
mercury reduces the reactivity by giving uniformity to the surface.
Very pure zinc reacts readily with dilute acids if previously coated
with copper by adding copper(II) sulphate:

Cu2+ +Zn-+Zn2+ + Cuj

This zinc-copper couple reacts with methanol, the mixture
reducing an alkyl halide to an alkane:

Zn + CH 3 OH + C 2 H 5 I -> Zn^2 + + CH 3 O~ + I" + C 2 H 6