Cl has higher electron affinity than F. This is due to small size of fluorine. As a result of its very small size, the inter-electronic repulsion is relatively compact in 2p sub-shell of chlorine atom. On comparing chloride ion, with fluoride ion we find that electron density per unit volume in fluoride ion (F- ) is more than in chloride (CI --) ion. This means that coming electron in fluorine atom finds less attraction than in chlorine atom. Consequently, electron affinity of chlorine is higher than that of fluorine.
