The ions S-2, Cl-1, K+, Ca+2, Sc+3 have the same total number of electrons as the noble gas argon. How would you expect the radii of these ions to vary?

Cl^- <S^2-<Sc^3+ <Ca^2+<K^+

Explanation:

We know that ionic radius of ions decreases from right to left in the periodic table.  This is because, ionic radii decreases with increase in nuclear charge. This explains why; Sc^3+ <Ca^2+<K^+.

Secondly, even though Cl^- is isoelectronic with S^2-, the size of the nuclear charge in Cl^-  is larger compared to that of  S^2- . Hence Cl^-  is smaller than S^2- in ionic radius owing to increased nuclear attraction in Cl^-.