Down a group, electron affinity typically decreases. This is because the atomic radius increases down a group. Fluorine, which is higher up the group then chlorine, has a lower electron affinity. This is because the electrons in the outermost shell of a fluorine atom are closer together. The electron gained also feels a great amount of repulsion from the electrons originally in the outermost shell. Energy is required to keep the gained electron in the shell, causing fluorine to have a smaller electron affinity than chlorine. Hope this helps :)
