CBSE Class 11-science Answered

Oxygen and magnesium are isoelectronic species i.e.  they have same number of electrons but differ in the magnitude of their nuclear charge.

Mg⁺² ion has 12 protons in its nucleus and therefore exert higher effective nuclear charge compared to O^2- which has 8 protons.

As a result, the inner shells and the valence shell electrons are more strongly attracted by the nucleus in Mg⁺².

Therefore Mg⁺² ion is smaller in size than O^2- ion although both have same electronic structure.