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Q.
Which of the following has the most negative value of electron gain enthalpy?
Classification of Elements and Periodicity in Properties
Solution:
In general, electron affinity decreases down the group but electron affinity for chlorine is more than fluorine, and similarly electron affinity for sulphur is more than oxygen, because in $F$ and $O$, due to small size of the atom, the electrons are already crowded. Entry of one more electron results in more repulsions, which leads to absorption of some energy, so the energy released is less than the expected.