Question

Which of the following has the smallest size?

• A. Na⁺
• B. Mg⁺²
• C. Cl^-
• D. F^-

Answer 1

The correct option is B

Mg⁺²

As the nuclear charge per $e^{-}$ is maximum in $M{g}^{2+}$, it has smallest size among $N{a}^{+},M{g}^{2+},C{l}^{-}$ and $F^{-}$.

To make it more elaborate:

$N{a}^{+}andM{g}^{2+}$ have only 2 outer shells so they are smaller.

Now between $M{g}^{2+}$ and $N{a}^{+}$, $M{g}^{2+}$ has a higher number of protons, so higher effective nuclear charge.