Modern inorganic chemistry

GROUP VI 285

BINARY COMPOUNDS

The elements (X) in this group are two electrons short of a noble

gas structure which they can achieve either by gaining or sharing

electrons. The formation of the X^2 ~ ion may require considerable

amounts of energy ; thus for oxygen 650 kJ must be supplied for the

reaction

Despite this energy requirement, many solid ionic oxides are

known because, in their formation, a high lattice energy results from

the combination of a metal cation with the small, double-charged

O^2 ~~ ion, and this provides the energy required. (In aqueous solution,

many ionic oxides are insoluble ; if the oxide is soluble, then since

O^2 " is a very strong base (p. 89) it reacts with the water to give

hydroxide ions OH~.) In contrast to the oxide ion, the larger ions

S^2 ~, Se^2 " and Te^2 ~ produce smaller lattice energies with cations

in solids, and only the most electropositive metals yield ionic solids

containing these anions ; the other elements give essentially covalent

compounds.

Oxygen bonds covalently to many non-metals, and in many

oxides, both with metals and non-metals, the other element achieves

a high oxidation state, for example

CrO 3 ( + 6), SO 3 ( + 6), Cl 2 O 7 ( + 7)

(This ability to bring out high oxidation states is exhibited also by

fluorine; it is to be attributed to the high electronegativities of

oxygen and fluorine.)

OXIDES

Oxygen will unite with, i.e. oxidise (in the simplest sense), most
elements other than the noble gases, forming oxides. With strongly
electropositive metals, for example sodium or calcium, the oxides
formed are ionic, for example sodium gives the oxide Na 2 O, con-
taining the ion O^2 ". Such oxides are basic, reacting with acids to
give salts and water only; many examples are given in this book.
With less electropositive metals or elements, for example aluminium,
zinc, lead, the bond between element and oxygen may assume a
partly covalent character, and the oxide becomes amphoteric,
dissolving in both acids and bases, for example

A1 2 O 3 + 6H+ 4- 9H 2 O -» 2[A1(H 2 O) 6 ]^3 +

hvd ruled