Modern inorganic chemistry

THE TRANSITION ELEMENTS 409

utilise a copper-protein complex called hemocyanin* as an oxygen

carrier, analogous to haemoglobin in mammals.

Copper differs in its chemistry from the earlier members of the

first transition series. The outer electronic configuration 3dl°4sl

contains a completely-filled set of d-orbitals and, as expected,

copper forms compounds where it has the oxidation state +1,

losing the outer (4s) electron and retaining all the 3d electrons.

However, like the transition metals preceding it, it also shows the

oxidation state + 2; oxidation states other than +1 and + 2 are

unimportant.

The metal melts at 1356 K and oxidises at red heat in air to give

the black +2 oxide CuO; at higher temperatures the red-yellow +1

oxide Cu 2 O is obtained. In dry air, little corrosion occurs, but in

the ordinary atmosphere a green film slowly forms, and this protects

the metal from further corrosion (hence its use in roofing). The

composition of the green film varies; normally it is a basic carbonate

of copper, but near the sea basic chloride is also a component and

in industrial areas a basic sulphate is found. Copper is readily

attacked by halogens and by sulphur on heating. Since

Cu2+(aq) + 2e~ -» Cu(s):£^ = +0.34 V
copper is not attacked by water or by dilute non-oxidising acids to
give hydrogen. It is attacked by nitric acid, to give a solution of
copper(II) nitrate Cu(NO 3 ) 2 and oxides of nitrogen, the nature of
the latter depending on the concentration of acid (dilute gives
nitrogen monoxide, concentrated the dioxide). In concentrated
sulphuric acid, some copper(II) sulphate is formed in solution, and
sulphur dioxide is evolved, but other products (for example sulphur,
copper(II) sulphide) may also be formed, and the reaction is un-
suitable for preparative purposes.
Copper is precipitated on the surface of some metals which reduce
it from an aqueous solution of its + 2 salts, for example

Fe 4- Cu2+(aq) -> Cu + Fe^2 + (aq)

Oxidation state + 2

In this oxidation state with nine d electrons, copper compounds are

usually coloured and paramagnetic.

COPPER(II) HALIDES, CuX 2

The anhydrous fluoride CuF 2 is white, the chloride yellow and the

bromide almost black; in the crystal of the chloride, each copper