Question

Why chlorine has more negative electron affinity fluorine?

Answer 1

Yes, fluorine is one of the exceptions -- and the element with the highest electron affinity is chlorine (note that the most electronegative element is fluorine however). 

The reason that the electron affinity is not as high as might otherwise be predicted for fluorine is that it is an extremely small atom, and so it's electron density is very high. Adding an additional electron is therefore not quite as favorable as for an element like chlorine where the electron density is slightly lower (due to electron-electron repulsion between the added electron and the other electrons in the electron cloud).

Answer 2

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)?