The experimental value of the electron affinity is found to be larger for chlorine than for fluorine. This is surprising since the vacancy in chlorine is in a 3p subshell compared with the 2p vacancy in fluorine, and is thus further removed from the attractive force of the nucleus. A possible explanation for this reversal of the expected order might be the greater increase in the average electron-electron repulsion energy for F ®  F^- than for Cl ®  Cl^- since the fluorine atom is smaller than the chlorine atom. The overall energy of the process is determined by the energy decrease due to the binding of the extra electron (as determined by the effective nuclear charge and favouring fluorine over chlorine) and the energy increase due to the repulsion of the added electron by those already present. The repulsion effect should be smaller for the Cl atom since its charge density is more diffuse. The net decrease in energy can therefore be greater for Cl than for F even though F possesses the larger effective nuclear charge.