Which of the following elements has the highest electron gain enthalpy (most negative): $\text{F}$, $\text{Cl}$, $\text{Br}$, $\text{I}$?

Solution

1. The electron gain enthalpy generally becomes less negative down the group.
2. However, $\text{F}$ has a very small atomic size leading to strong inter-electronic repulsions.
3. This makes the addition of an electron to $\text{F}$ less exothermic than to $\text{Cl}$.
4. Thus, $\text{Cl}$ has the most negative electron gain enthalpy among halogens.
5. Hence the answer is (B).