This is a classic exception in periodic trends.

• Fluorine ($F$) is the most electronegative, so one might expect it to have the highest electron gain enthalpy (electron affinity).
• However, Chlorine ($Cl$) actually has a higher (more negative) electron affinity value.
• Reason: Fluorine's atom is extremely small. When an electron is added to the small $2p$ subshell of $F$, it experiences significant electron-electron repulsion from the electrons already present.
• Chlorine's $3p$ subshell is larger, accommodating the new electron with less repulsion.