Trends In Periodic Table

Electron affinity gradually increases on moving down the group but due to smaller size of fluorine, the added extra electron experience strong force of repulsion, hence its electron affinity is lesser than that of chlorine.
The order of electron affinity of halogens is

The correct order of electron gain enthalpy (with sign) of the halogen atom is The reason that fluorine has a less negative electron affinity than chlorine is relatively small size of F atom resulting in the repulsions (electron-electron) to the entering electron.