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The species Ar, K+ and Ca2+ contain the same number of electrons. In which order do their radii increase?

1. Ca2+ < Ar < K+
2. Ca2+ < K+ < Ar
3. K+ < Ar < Ca2+
4. Ar < K+ < Ca2+

Correct answer: Ca2+ < Ar < K+

Solution

All three species have the same number of electrons (18), but their nuclear charges differ. Higher nuclear charge pulls electrons closer, reducing size. Thus, Ca2+ (20 protons) < Ar (18 protons) < K+ (19 protons) in size.

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