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

Explanation

Atomic radii decrease across a period. Cations are smaller than their parent atoms. Among isoelectronic ions, the one with the large positive nuclear charge will have a smaller radius.
Hence the largest species is Mg, the smallest one is `Al^(3+)` The size of an anion greater while that of the cation is smaller than that of its parent atom, e.g. `F^(-) = (1.36 Å) gt F (= 0.72 Å) , Cl^(-) (=1.81 Å) gt Cl (=0.99 Å) , Na^(+) =(0.95 Å) lt Na (=1.90 Å) , Ca^(2+) (=0.99 Å) lt Ca(=1.97Å)`