Question

The sizes of following species increase in the order:

• A. Mg²⁺ < Na⁺ < F^- < Al
• B. F^- < Al < Na⁺ < Mg²⁺
• C. Al < Mg²⁺ < F^- < Na⁺
• D. Na⁺ < Al < F^- < Mg²⁺

Answer 1

The correct option is A Mg²⁺ < Na⁺ < F^- < Al

For isoelectronic species, $M{g}^{2+}$, $N{a}^{+}$, $F^{-}$ the sizes increase in the order.
$M{g}^{2+}$ $<$ $N{a}^{+}$ $<$ $F^{-}$ $<$ Al

It is easier to deduce $M{g}^{2+}$ $<$ $N{a}^{+}$ $<$ $F^{-}$ because in the first 2 there is a loss of an entire shell and in the third, there is an addition of an electron keeping the nuclear charge the same.

It gets slightly trickier with $F^{-}$ and Al.The atomic radius of Al (143pm) is greater than ionic radius of $F^{-}$(136 pm). Instead of remembering this data, we follow the general rule that the addition of a new shell usually outweighs the addition of protons. So, the size of Al would be larger..