Question

The species Ar, K⁺ and Ca²⁺ contain the same number of
electrons. In which order do their radii increase ? [2015]
(a) Ca²⁺ < Ar < K⁺ (b) Ca²⁺ < K⁺ < Ar
(c) K⁺ < Ar < Ca²⁺ (d) Ar < K⁺ < Ca²⁺

Answer 1

Explanation:

The order do their radii increase is option correct

(a) Ca²⁺ < Ar < K⁺

Answer 2

- the correct order of the ionic radii of the given ions is : Ca²⁺ < K⁺ < Ar

- the atoms and ions given in the above question are isoelectronic, i.e., they have the same number of electrons

- for the isoelectronic species, with the increase of the positive charge the size of cation decreases

- therefore, the correct order of their ionic radii is Ca²⁺ < K⁺ < Ar

- so the correct option is (b)