Question

Which of the following has least ionisation energy?

• A. $O^{2-}$
• B. $N^{3-}$
• C. $M{g}^{2+}$
• D. $F^{-}$

Answer 1

Answer: (B)

$N^{3-}$

${}_{8}O:1s^{2}2s^{2}2p^4$ (Ion with least $IE$ is $⟶N^{3-}$ as $N$ being less electronegative than $O$ and also the outermost shell is $S$

$O^{2-}:1s^{2}2s^{2}2p^2$ of low energy. Also, the radii of $N^{3-}$ is bigger than $O^{2-}$. Hence has low $IE$)

${}_{7}N:1s^{2}2s^{2}2p^3$

$N^{3-}:1s^{2}2s^2$

$Mg:1s^{2}2s^{2}2p^{6}3s^2$

$M{g}^{2+}:1s^{2}2s^{2}2p^6$

$F:1s^{2}2s^{2}2p^5$

$F^{-}:1s^{2}2s^{2}2p^6$