Which of the following elements has the highest negative electron gain enthalpy?

Solution

1. Electron gain enthalpy generally becomes more negative across a period.
2. However, $\text{F}$ has a smaller size than $\text{Cl}$, leading to significant inter-electronic repulsions in its $2p$ subshell.
3. This makes the addition of an electron to $\text{Cl}$ more exothermic than to $\text{F}$.
4. Thus, $\text{Cl}$ has the highest negative electron gain enthalpy among the options.
5. Hence the answer is (B).