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  4. Which of the following has the highest electron affinity?

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Q. Which of the following has the highest electron affinity?

AFMCAFMC 2010Classification of Elements and Periodicity in Properties

Solution:

As the size of atom decreases, a larger amount of energy is released when an electron is added to an isolated gaseous atom. Among the given species, the size of $O$ atom is smallest ( $ \because $ size of parent atom is smaller as compared to its anion), thus its electron affinity is highest.
[Remember! the value of second electron affinity is generally positive. This is because, the second electron experience a greater force of repulsion from the existing electrons. Hence, to overcome this repulsion, some heat is absorbed. This is also the reason for lower electron affinity of $ F^{-} $ as compared to $O$.]