The cumulative effect of the contraction of
the lanthanoid series, known as lanthanoid
contraction, causes the radii of the members
of the third transition series to be very similar
to those of the corresponding members of the
second series 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