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

Explanation

The most stable oxidation state of langthanides 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.