The #"lanthanides"# and #"actinides"# each show progressive filling of the #f-"orbitals"#. We know that the filling of the #d-"orbitals"# gives rise to peculiar chemistry in the transition series, given the geometric properties of the #d-"orbitals"#. The #f-"orbitals"# are multi-lobed, and they are very poorly shielded from the nuclear charge.
And thus #f-"orbital"# electrons do not tend to participate in bonding because they are too tightly held by the nuclear charge. The result is that the lanthanides tend to lose the 2 or 3 valence electrons in the s shell, and form a series of #M^(3+)# cations (or more rarely #M^(2+)# cations). Lanthanides and actinides thus tend to have a particular chemistry which is exclusive to themselves. Most lanthanide chemists also study the chemistry of scandium and yttrium, which as #M^(3+)# and #M^(2+)#, very potent Lewis acids, have a parallel and complementary chemistry.
