Why `Sm^(2+) , Eu^(2+) and Yb^(2+)` ions are good reducing agents , but and aqueous solution of `Ce^(4+)` is good oxidsing (Z=64) and Lutetium (Z= 7) are especially, stable . Why ?

Answer

The most stable oxidation state of lanthanoides is +3. Hence , in +2 state tent to change to +3 state by loss of electrons and those in +4 state tend to change to +3 state by gain of electrons.