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 Thus, the formation of Ce
IV is favoured by its
noble gas configuration, but it is a strong oxidant reverting to the
common +3 state