The minimum amount of energy required to remove an electron from an atom of element in its gaseous state is called 1st ionization energy The SI unit of ionization energy is kilojoules per molekJmole1 Ionization energy depend on atomic radius The higher the atomic radius the less effective the nuclear attraction on outer electronsenergy level and thus the lower the ionization energy For alkali metals the 1st ionization energy decrease down the group as the atomic radius increase and the effective nuclear attraction on outer energy level electrons decrease