Question

Which of the following species has the highest electron affinity?

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

Answer 1

The correct option is B $O$

Energy is released when an electron is added to the atom. Hence 1st electron affinity $\left(E{A}_1\right)$ is always taken as negative. The addition of second electron to the negativity charged ions is opposed by Coulombic repulsion, hence energy has to be supplied for the addition of second electron. Hence $\left(E{A}_2\right)$ is taken as positive.

Addition of an electron to $F^{-}$ and $O^{-}$ ions will involve repulsions, hence their $\left(E{A}_2\right)$ will be positive.
Due to stable fully filled configuration of ${Na}^{+}$ $\left(1s^{2}2s^{2}2p^6\right)$ it has lower value of electron affinity than $O$. Hence $O$ has highest electron affinity.