The species Ar, K+ and Ca2+ contain the same number of electrons. In which order do their radii increase ?
- Ca2+ < K+ < Ar
- K+ < Ar < Ca2+
- Ar < K+ < Ca2+
- Ca2+ < Ar < K+
Answer
(1) Ca2+ < K+ < Ar
Solution
Ar, K+ and Ca2+ are isoelectronic (18 electrons each).
Nuclear charge: Ar (18) < K+ (19) < Ca2+ (20)
∵ For isoelectronic species, greater nuclear charge ⇒ stronger attraction on the same number of electrons ⇒ smaller radius.
∴ Radii increase as Ca2+ < K+ < Ar
Answer: (1)