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Q.
Arrange S , O , and Se in ascending order of electron affinity
Classification of Elements and Periodicity in Properties
Solution:
$S , O$ and Se are all $16^{\text {th }}$ group elements.
Since electron affinity generally decreases down the period. So, the expected order would be $O > S > Se$
But this is not the observed case. The reason for this is that the oxygen atom is extremely small.
Therefore it cannot hold a negative charge in such a small space. Thus, its electron affinity is the lowest in its group. So, the correct order will be: $O <$ Se $< S$