Question
$A{l^{3 + }}$ has a lower ionic radius than $M{g^{2 + }}$ because
For iso-electronic species, size depends upon the effective nuclear charge and size is inversely proportional to the effective nuclear charge. More is the positive charge, more is the effective nuclear charge and thus, lesser is the size.
Since the effective nuclear charge is greater in $Al ^{3+}$ than $Mg ^{2+}$, it has a smaller size than $Mg ^{2+}$.
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