As we know that I.E is the minimum amount of energy required to remove an electron from the valence shell of an isolated gaseous atom in its ground state resulting in the formation of ion whereas, electron gain enthalpy is the amount of energy associated with the gain of electrons by the isolated gaseous atom. So,it is opp. of I.E Hence,if I.E of Na=5.1 its Electron Gain enthalpy will be -5.1