Modern inorganic chemistry

52 STRUCTURE AND BONDING
positive end of another. As a result of these attractive forces, chloro-
methane (molecular weight 50.5) has a melting point of 174.5 K,
well above that of methane (molecular weight 16, m.p. 89 K) and
also well above the hydrocarbon butane which has a molecular
weight comparable with it (molecular weight 58, m.p. 138 K). In
solid chloromethane, unlike solid methane, there is also evidence of
orientation of the molecules packed together in the crystal.

HYDROGEN BONDING

Figure 2.5 shows the boiling points of the hydrides in elements of

Groups IV, V, VI and VII. Clearly there is an attractive force be-

tween the molecules of the hydrides of fluorine, oxygen and nitrogen

Period

Figure 2.5. B.p. of hydrides in Groups IV to VII

in addition to the expected van der Waal's forces. This force, what-
ever its origin, is virtually absent in the hydrides of all but the three
elements named. The absence of the force in methane indicates that
the presence of at least one lone pair of electrons is essential, but
this attractive force is not found in the hydrides of larger elements
in the same group, which do have lone pairs of electrons.
The attractive force is called hydrogen bonding and is normally
represented by a dotted line, for example A—H • • • A—H; it is this