ΓΌ  Electron Gain Enthalpy (∆egH): It is the enthalpy change when an electron is added to an isolated gaseous atom. X(g) + e– → X– (g). Its unit is kJ/mol.
Greater the amount of energy released, higher is the electron gain enthalpy of the element.
It may be positive or negative depending on the nature of the element. For most of the elements, energy is released when electron is added to their atoms. So ∆egH is negative. Noble gases have large positive electron gain enthalpy because of their completely filled (stable) electronic configuration.



Variation along the Period: From left to right across a period, ∆egH become more negative. This is because of decrease in atomic radius and increase in nuclear charge. So, the ease of addition of electron increases and hence the ∆egH.
Variation down the Group: As the size increases, tendency to add the electron decreases hence electron gain enthalpy becomes less – ve.

Electron gain enthalpy of fluorine is less negative than chlorine. This is because, when an electron is added to F, it enters into the smaller 2nd shell. Due to the smaller size, the electron suffers more repulsion from the other electrons. But for Cl, the incoming electron goes to the larger 3rd shell. So, the electronic repulsion is low and hence Cl adds electron more easily than F.

Thus, in modern periodic table, alkali metals have the least –ve ∆egH and halogens have the most –ve ∆egH. Among halogens, the negative ∆egH decreases as follows.  Cl> F > Br > I. The negative electron gain enthalpy is also called electron affinity.

Noble gases have positive electron gain enthalpy. They have completely filled orbitals. Additional electron will be placed in next higher shell. As a result, energy has to be supplied to add an additional electron.

Formation of O is exothermic but O is endothermic.
O (g) + e-  →  O- (g);  ∆egH = - 141 KJ mol      
O- (g) + e-  → O2- (g);  ∆egH = + 780 KJ mol     


∆egH After the addition of one electron the atom becomes negatively charged and second electron is to be added to a – vely charged ion. Addition of second electron is opposed by electrostatic repulsion, hence energy has to be supplied for addition of second electron to overcome the strong electrostatic -repulsion between the negatively charged O ion and second electron being added.