Question

Arrange the following isolectronic species in increasing order of their ionic radii

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

Answer 1

$$\begin{array}{ccc}\text{Ion}& \text{Number of protons}& \text{Number of electrons}\\ O^{2-}& \text{8}& \text{10}\\ F^{-}& \text{9}& \text{10}\\ N{a}^{+}& \text{11}& \text{10}\\ A{l}^{3+}& \text{13}& \text{10}\end{array}$$
Higher the charge on a cation, stronger is the nuclear pull on same number of electrons. Hence, smaller is the ionic radius.

Higher the charge on an anion, stronger are the electronic repulsions. Hence, larger is the ionic radius.