o Electronic configurations of Na⁺ - 1s² 2s² 2p⁶ (less in one electron Na - [Ne] 3s¹ ) , therefore having 10 electrons

which resembles both Mg²⁺ - 1s² 2s² 2p⁶ (less in 2 electrons , Mg – [Ne] 3s²) and

O²– - 1s² 2s² 2p⁶ (gain in 2 electrons , O – [He] 2s² 2p⁴ ) both having equal 10 electrons .

Hence , both (i) Na⁺, Mg²⁺ and iii) Na⁺ , O²– are example of isoelectronic electron pairs.

^o Among other 2 options , (ii) Al³+ ([Ne] less in 3 electrons having total 10 electrons ), O– ( [He] 2s² 2p⁵ having total 9 electrons) and

(iv)N^3- ( [He] 2s² 2p⁶ a total of 10 electrons ), Cl^- ([Ne] 3s² 3p⁶ a total of 18 electrons) they are incorrect options.