GROUPS I AND II 127
This reaction is due to the very strong basic property of the hydride
ion H~ which behaves as a powerful proton acceptor and is therefore
strongly basic, i.e.H~ + H 2 O->H 2 t + OH^When the molten ionic hydrides are electrolysed, all yield hydrogen
at the anode, the metal at the cathode.
The hydrides of Group I, especially lithium hydride, react with
the hydrides of trivalent metals of Group III to form interesting
complex hydrides, probably the most important being lithium
aluminium hydride (lithium tetrahydridoaluminate) LiAlH 4 , well
known as a reducing agent in organic chemistry.
The hydrides of beryllium and magnesium are both largely
covalent. magnesium hydride having a 'rutile' (p. 36) structure,
while beryllium hydride forms an electron-deficient chain structure.
The bonding in these metal hydrides is not simple and requires an
explanation which goes beyond the scope of this book.THE HALIDES
Group I metals combine directly with all the halogens. The reactions
are exothermic, the greatest heats of formation being found when
the elements combine with fluorine. Except for the formation of the
fluorides, the heat of formation of a given halide increases as the
group is descended and the ionisation energies of the metallic
elements fall. The reverse is true for the fluorides, and the heat of
formation falls as the group is descended. This is due to the high
lattice energies produced from the 'combination' of the small
fluoride anion and the metal cation (p. 74). (Similar variations are
also noted with other small anions, for example nitride, carbide.)
All the Group I halides can be regarded as ionic*, this fact being
reflected in their high m.p. and b.p. and the ability of the melt to
conduct electricity. AH except lithium fluoride are soluble in water,
the insolubility of the lithium fluoride being a result of the high
lattice energy, which is sufficiently large to more than compensate
for the high hydration energies of the lithium and fluoride ions
(p. 78). Group II metals also form halides by direct combination.
The trends in heat of formation and m.p., however, whilst following
the general pattern of the corresponding Group I compounds, are
not so regular.
* Lithium bromide and iodide probably have some degree of covalency but this
does not affect the general conclusion.