AIEEE 2012 Solved Paper

Iso-electronic ions have same number of electrons. So, for iso-electronic ions, number of electrons = constant.

∴σ (Slaten's Constant) = Constant. As σ depends on number of electrons. If a element's number of electrons increases then that element's σ increases.

Also we know, Z‌effective ‌=Z−σ
As for iso-electronic ions, σ (Slaten's Constant) = Constant. So Z‌effective ‌ depend on only value of Z. If Z of an ion increases then Z‌effective ‌ also increases and if Z of an ion decreases then Z‌effective ‌ also decreases. And when Z‌effective ‌ increases then nuclear attraction towards outermost electrons increase and size of ion decreases. Similarly when Z‌effective ‌ decrease then nuclear attraction towards outermost electrons decreases and size of ion increases.
∴ Size ∝‌

1

Z‌eff or ‌Z

	Ca2+	K+	Cl−	S2−
Z	20	19	17	16

∴ Ionic radius order
Ca2+<K+<Cl−<S2−