The values of rmetal listed in Table 5.2 refer to 12-coordinate
metal centres; since not all metals actually adopt structures
with 12-coordinate atoms, some values of rmetal have
been estimated. The need for a consistent set of data is
obvious if one is to make meaningful comparisons within a
periodic sequence of elements. Values of rmetal (Table 5.2)
increase down each of groups 1, 2, 13 and 14. In each of
the triads of the d-block elements, rmetal generally increases
on going from the first to second row element, but there is
little change on going from the second to third row metal.
This latter observation is due to the presence of a filled 4f
level, and the so-called lanthanoid contraction (see Sections
22.3 and 24.3).