`Mg^(2+)` is smaller than `O^(2-)` in size, throgh both have same electronic configuration. Explain?

Explanation

`Mg^(2+)` and `O^(2-)` both are isoelectronic i.e., have same under of electrons. But `Mg^(2+)` having `12` protons in its nucleus exerts higher effective nuclear charge than `O^(2-)` having `8` protons and thus valence sheel as well as inner sheels electrons are more strongly attracted by the nucleus in `Mg^(2+)` resulting smaller size than `O^(2-)`.