Variability in the oxidation states of lanthanides is limited. Why? 

Explanation

The common oxidation state of lanthanides is `+3`. Existence of `+2 and +4` states is due to the fact that empty, half-filled or completely f-subshells provide lower energy and ions get stabilised. `Ce^(4+)` has `f^(0), Tb^(4+)` ha `f^(7), Eu^(2+)` has `Yb^(2+)` has `f^(14)` stable configurations.