Why does chlorine have a higher electron affinity than fluorine? Since fluorine has its valence electrons in the n=2 energy level, and since chlorine has its valence electrons in the n=3 energy level, one would initially expect that an electron rushing towards fluorine would release more energy, as it would land in the n=2 energy level, whereas in chlorine, the electron would land only in the n=3 energy level, and would then not release as much energy.  Thus, one would expect fluorine to have a greater electron affinity than chlorine.  However, why is it that chlorine has a higher electron affinity (349 kJ/mol)  than fluorine (328.165 kJ/mol)?
Fluorine, though higher than chlorine in the periodic table, has  a very small atomic size. This makes the fluoride anion so formed unstable (highly reactive) due to a very high charge/mass ratio. Also, fluorine has no d-orbitals, which limits its atomic size. As a result, fluorine has an electron affinity less than that of chlorine.

See this.