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.