Arrange the following isoelectronic species `(O^(2-) , F^(-) , Na^+, Mg^(2+))` in order of:
increasing ionization energy

Explanation

`O^2 lt F lt Na^+ lt Mg^(2+)`. As dictated above, greater the positive charge, smaller the size of the cation and hence closer the valence electrons to the nucleus. Increased effective nuclear charge results in the valence electrons being tightly bound to the nucleus causing the ionization energy to increase