Which of the following species will have the largest and the smallest size `Mg, Mg^(2+), Al, Al^(3+)`?

Explanation

(i) Mg and A1 belong to the third period .Across a period atomic radii decrease due to increases nuclear charge . Therefore atomic size of Al is smaller than that of Mg.
(ii) Further cations are smaller than their parent atoms. Therefore `Mg^(2+)` si smaller than Mg and `Al^(3+)` is smaller than Al.
(iii) `Mg^(2+)` and `Al^(3+)` are isoelectronic ions. Among isoelectronic ions, higher the + ve charge smaller the size. Therefore ionic radius of `Al^(3+)` is smaller than that of `Mg^(2+)`
From the above discussions it follows that Mg has the largest while `Al^(3+)` has the smallest size.