Question

Which of the following isoelectronic ions have lowest ionization?

• A. $N{a}^{+}$
• B. $M{g}^{2+}$
• C. $F^{-}$
• D. $O^{2-}$

Answer 1

The correct option is D $O^{2-}$

It is easier to eject an electron from negatively charged species than a positively charged or neutral species. Higher the negative charge, easier is the process.
Therefore, $O^{2-}$ has lowest ionisation energy.