The almost identical radii of Zr
(160 pm) and Hf (159 pm), a consequence of
the lanthanoid contraction, account for their
occurrence together in nature and for the
difficulty faced in their separation In the lanthanoids, La(II) and Ln(III) compounds are predominant
species However, occasionally +2 and +4 ions in solution or in solid
compounds are also obtained This irregularity (as in ionisation
enthalpies) arises mainly from the extra stability of empty, half-filled
or filled f subshell