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^-.
