Question

Which of the following species will have the largest and the smallest size?
$Mg,{Mg}^{+2},Al{Al}^{+3}$

Answer 1

(i) $Mg$ and $Al$ belong to the third period. Across a period , atomic radii decrease due to increased nuclear charge. Therefore, atomic size of $Al$ is smaller than that of $Mg$.
(ii) Further cations are smaller than their parent atom. Therefore , $M{g}^{2+}$ is smaller than $Mg$ and $A{l}^{3+}$ is smaller than $Al$.
(iii) $M{g}^{2+}$ and $A{l}^{3+}$ are isoelectronic ions. Among isoelectronic ions, higher the +ve charge, smaller the size. Therefore , atomic radius of $A{l}^{3+}$ is smaller than that of $M{g}^{2+}$.
From the above discussion, it follows that $Mg$ has the largest while $A{l}^{3+}$ has the smallest size.