Modern inorganic chemistry

114 HYDROGEN

the others are endothermic and plumbane PbH 4. the last hydride in
the group, is almost too unstable to exist at all. (We shall note some
of the methods needed to prepare these less stable hydrides in later
chapters.) Since the stability of the typical hydride (i.e. that in which
the element shows its group valency) falls off. it is hardly surprising
to find that the lower elements in a group do not form families of
hydrides (for example, in Group IV carbon and silicon form
numerous hydrides, germanium forms a few. tin forms one (stannane.
SnHJ and lead just manages to form PbH 4 ).
The most important trend to be noted in the covalent hydrides is
the change in acid-base behaviour as we cross a period from
Group IV to Group VII. In Period 1, we have

CH 4 NH 3 H 2 O HF

no acidic or basic basic acidic

basic properties (very weakly acidic) and acidic (weakly basic)

This change in properties cannot be simply accounted for in terms
of bond energies; the mean X—H bond energy increases from
nitrogen to fluorine, and hydrogen fluoride has a large bond-
dissociation energy (566kJmol~^1 ). But we note that in the CH 4
molecule there are no lone pairs of electrons—all four valency
electrons are involved in bonding. In ammonia, there is one lone
pair, which as we have seen can be donated either to a proton
(making ammonia a Lowry-Br0nsted base, NH 3 + H + ^NH^)
or to another acceptor molecule (making ammonia a Lewis base,
p. 91). The molecules H 2 O and HF have two and three lone pairs
respectively; falling-off of base strength implies that the presence of
more than one lone pair reduces the donor power of the molecule.
But, obviously, the appearance of acidic behaviour implies that the
bond X—H is more readily broken heterolytically i.e. to give X~ +
H +. We may ascribe this to polarity of the bond, i.e. by saying that
the pair of electrons in the covalent H—F bond is closer to the
fluorine than to the hydrogen. Unfortunately, there is no very sure
method of ascertaining this bond polarity (the fact that hydrogen
fluoride HF has a dipole moment means that the molecule as a

• —
  whole is polar in, presumably, the sense H—F, but this does not
  necessarily tell us about the bond polarity). Another way of des-
  cribing this trend towards acidity is to say that the electronegativity
  of the element increases from carbon to fluorine. We may simply
  note that this trend to acidity is also apparent in other periods, for
  example, in Period 3, silane SiH 4 is non-acidic and non-basic.