Modern inorganic chemistry

HYDROGEN 113

these metal hydrides or interstitial hydrides may have variable

composition (for example TiHx 7 ), depending on the uptake of

hydrogen, i.e. they are non-stoichiometric. One further property in

particular distinguishes these metal hydrides from the ionic hydrides;

in the latter, uptake of hydrogen is not only quantitative but causes

a contraction, i.e. the centres of the metal atoms (which become

Figure 5.1. Interstitial positions between layers of metal atoms

cations) move closer together—the metal lattice is, as it were, drawn

together. In the metal hydrides, there is no such contraction, and,

indeed, the metal atoms may move apart slightly. Hence formation

of an ionic hydride leads to an increase in density, but formation of

a metal hydride causes a decrease in density.

REACTIONS WITH NON-METALS AND WEAKLY

ELECTROPOSITIVE METALS

Most of the elements of Groups III to VII form hydrides which are
essentially covalent. Some examples are Group IV, methane CH 4 ;
Group V, phosphine PH 3 ; Group VI, hydrogen sulphide H 2 S;
Group VII, hydrogen chloride, HC1. There are several points to
notice about these covalent hydrides. First, they are nearly all
volatile liquids or gases; but the simple hydrides NH 3 , H 2 O and HF,
formed from the head elements of Groups V-VI1, show hydrogen
bonding characteristics which make them less volatile than we
should expect from the small size of their molecules (p. 52).
Secondly, the ability to form more than one hydride falls off as
we go across a period. Thus, in Period 1. boron and carbon both
form whole families of hydrides, nitrogen forms three (ammonia.
NH 3 ; hydrazine. N 2 H 4 ; hydrazoic acid. N 3 H). oxygen two (H 2 O.
H 2 O 2 ) and fluorine one (HF). Again, as we descend a group, the
energetic stability of the hydrides decreases—indeed, many hydrides
are endothermic. and need indirect methods to supply the necessary
energy for their preparation. In Group IV, methane is exothermic,