Assertion (A) : $\mathrm{Na}^{+}$and $\mathrm{Mg}^{2+}$ ions are isoelectronic but the ionic radius of…
Assertion (A) : $\mathrm{Na}^{+}$and $\mathrm{Mg}^{2+}$ ions are isoelectronic but the ionic radius of $\mathrm{Na}^{+}$is greater than that of $\mathrm{Mg}^{2+}$.
Reason (R) : The effective nuclear charge of $\mathrm{Na}^{+}$ion is less than that of $\mathrm{Mg}^{2+}$ ion.
Both (A) and (R) are correct but (R) is not the correct explanation of (A).
Both (A) and (R) are correct and (R) is correct
explanation of (A).
(A) is not correct but (R) is correct.
(A) is correct but (R) is not correct.
Solution
$\mathrm{Na}^{+}$and $\mathrm{Mg}^{2+}$ are isoelectrionic species having $10 e^{-}$ each. But $\mathrm{Mg}^{2+}$ has 12 protons while $\mathrm{Na}^{+}$has 11 protons. Hence effective nuclear charge is greater in case of $\mathrm{Mg}^{2+}$.
ionic radius $\propto \frac{1}{\text { effective nuclear charge }}$