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

Explanation

The atomic number of fluorine is nine. Its electronic configuration is `F(Z=9)1s^(2)2s^(2)2p^(5)`. But in case of chlorine, the electronic configuration of `Cl(Z=17)` is `1s^(2)2s^(2)2p^(6)3s^(2)2p^(5)`.
The atomic size of fluorine is less than that of chlorine. When the incoming electron comes in the electronic sphere of the fluorine atom, there occurs strong inter-electronic repulsion. Therefore, the incoming electron is less firmly held by the nucleus. As a result, electron affinity will be less. On the other hand, the size of chlorine is larger than fluorine. It can easily accommodate an extra electron in relatively larger 3p.subshell and have high value of electron affinity than fluorine.