Why electron affinity of fluorine is less than that of chlorine ?

Explanation

Electronic configuration of F (Z = 9) and CI (Z=17) are as follows.
`F(Z=9) : 1s^2 2s^2 2p_x^2 2p_y^2 2p_z^1`
`Cl(Z=17) : 1s^2 2s^2 2p^6 3s^2 3p_x^2 3p_y^2 3p_z^1`
Due to very small atomic size of F, strong inter electronic repulsions take place in relatively compact 2p-subshell and thus incoming electron is less firmly held by the nucleus. Therefore, F has a less negative electron gain enthalpy. On the other hand, Cl atom having larger atomic size can easily accomodate an extra electron in relatively larger 3p subshell and thus has a more negative electron gam enthalpy.