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Question:

Which of the following represents the correct order of increasing first ionization enthalpy for Ca, Ba, S, Se, and Ar?

Ba<Ca<Se<S<Ar

Ca<Ba<S<Se<Ar

Ca<S<Ba<Se<Ar

S<Se<Ca<Ba<Ar

Solution:

(i) In a group, the I.E decreases. Hence Ca > Ba and S > Se
(ii) In a period, I.E increases from left to right in general. Hence Ar > S and Se > Ca
Hence the order is Ar > S > Se > Ca > Ba and the answer is (B).