Modern inorganic chemistry

286 GROUP VI

A1 2 O 3 + 6OH~ + 3H 2 O -> 2[AI(OH) 6 ]^3 -

Notice that the acidic character is associated with the ability of

aluminium to increase its covalency from three in the oxide to six in

the hydroxoaluminate ion, [A1(OH) 6 ]^3 ~; the same ability to

increase covalency is found in other metals whose oxides are

amphoterie, for example

ZnO -> [Zn(OH) 4 ]^2 ~ or [Zn(OH) 6 ]^4 ~

PbO -» [Pb(OH) 4 ]^2 - or [Pb(OH) 6 ]^4 ~

HIGHER OXIDES

Variable oxidation state is also exhibited in the oxides themselves

among metals in this region of electronegativity. Thus lead, for

example, forms the monoxide PbO ( + 2) and the dioxide PbO 2

( + 4) (the compound Pb 3 O 4 is not a simple oxide but is sometimes

called a 'compound' oxide). Similarly, manganese gives the oxides

MnO and MnO 2.

Although the dioxides are oxidising agents, for example

PbO 2 + 4HC1 -> PbCl 2 + 2H 2 O + C1 2 T

the oxidising power lies in the higher valency or oxidation state of the
metal, not in the presence of more oxygen (distinction from peroxides,
see below).
The more noble metals (for example copper, mercury and silver)
can form oxides, and exhibit variable oxidation state in such
compounds (for example Cu 2 O, CuO), but it is not easy to prepare
such oxides by direct action of oxygen on the metal, and elevated
temperatures are necessary. Moreover, in the case of silver and
mercury, loss of oxygen from the oxide by heating is easy. The
oxides are, however, basic (for example Ag 2 O -> Ag+ , CuO -» Cu^2 +
in acids).

ACIDIC OXIDES

The other more electronegative elements are non-metals and form
oxides which are entirely covalent and usually acidic. For example,
sulphur yields the oxides SO 2 and SO 3 , dissolving in bases to form
the ions SOf ~ and SOj" respectively. A few non-metallic oxides
are often described as neutral (for example carbon monoxide and
dinitrogen oxide) because no directly related acid anion is known
to exist.