such that there are four environments with coordination
numbers of 12, 13 or 16. Atypical structures are also exhibited
by most of the p-block metals. In group 13, Al and Tl
adopt ccp and hcp lattices respectively, but Ga (the aform)
and In adopt quite different structures. Atoms of Ga
are organized so that there is only one nearest neighbour
(at 249 pm), with six next-nearest neighbours lying at distances
within the range 270 and 279 pm, i.e. there is a tendency
for the atoms to pair together. Indium forms a
distorted ccp lattice, and the twelve near neighbours separate
into two groups, four at 325pm and eight at 338 pm.† In
group 14, Pb adopts a ccp structure, but in white Sn (the
stable allotrope at 298 K), each atom possesses a coordination
number of only 6 (grey Sn, see Section 5.4). Metals
with coordination numbers of less than 8 are among those
that are the most volatile.