`Mg^(2+)` ion is smaller than `O^(-2)` ion although both have same electronic configuration. Why ?

Explanation

Although `Mg^(2+)` and `O^(2-)` ions are isoelectronic species. The size of cațion `(Mg^(2+))` is smaller than the anion `(O^(-2))`. This is because, if we compare the nuclear charge of both ions, then we find that `Mg^(2+)` cation have greater nuclear charge than the `O^(2-)` anion. So, in cation `(Mg^(2+))`, electrons are pulled towards the nucleus with greater force than in the anion `(O^(2-))`. As a result, size of `Mg^(2+)`.is smaller then the `O^(2-)`