The minimum amount of energy required to gainacquire an extra electron by an atom of element in its gaseous state is called 1st electron affinity The SI unit of electron affinity is kilojoules per molekJmole1 Electron affinity depends on atomic radius The higher the atomic radius the less effective the nuclear attraction on outer energy level electrons and thus the lower the electron affinity For halogens the 1st electron affinity decrease down the group as the atomic radius increase and the effective nuclear attraction on outer energy level electrons decrease Due to its small sizeatomic radius Fluorine shows exceptionally low electron affinity This is because a lot of energy is required to overcome the high repulsion of the existing and incoming electrons