Why `Sm^(2+) , Eu^(2+)` and `Yb^(2+)` ions in solution are good reducing agent but an aqueous solution of `Ce^(4+)` is a good oxidising agent?

Explanation

The most stable oxidation state of lanthanides is `+3`. Hence, ions in `+2` state tend to change to `+3` state by loss of electron and those in `+4` state tend to change to `+3` state by gain of electron.