In each of the following pairs,  which species has a larger size? Explain.
(i) K or K⁺ 
(ii) Br or Br^- 
(iii) O^2- or F^- 
(iv) Li⁺ or Na⁺ 
(v) P or As
(vi) Na⁺ or Mg²⁺

(i) K or K⁺: K atom has large size than K⁺ ion because the size of cation (or radius) is always smaller as compared to the parent atom.
(ii) Br or Br^- : Br^- ion has large size because it is an anion and it has always large size as compared to the parent atom.
(iii) O^2- or F^-: Both the anions are isoelectronic species. Since the nuclear charge of O^2-ion is small, its size is, therefore, more than that of F^-ion.
(iv) Li⁺ or Na⁺: Both the cations are formed from the atoms which are present in group 1. As Na atom is larger than Li atom, therefore, Na⁺ ion is also bigger in size than Li⁺ion.
(v) P or As: Both these atoms belong to group 15 of the periodic table. Since As is placed after P, therefore its size is large because atomic size increases down the group.
(vi) Na⁺ or Mg²⁺: Both the cations are isoelectronic in nature with 10 electrons each. Since the nuclear charge of Na⁺ ion is more than that of Mg²⁺ ion, its size is, therefore, large.