Question

Which of the following has the highest electron affinity?

• A. $F^-$
• B. $O^-$
• C. $O$
• D. $Na$

Answer 1

Answer: (C)

$O$

Answer 2

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 $O$.]