Just as energy (called ionization enthalpy) is required to remove an electron from an isolated gaseous atom so as to convert it into a positive ion, energy is released when an electron is added to an isolated gaseous atom so as to convert it into a negative ion. This energy is called electron gain enthalpy.
It may be defined as the energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous negative ion, i.e., anion. It is denoted by ΔegH.
This process may be represented as :
X(g) + e– —–> X–(g) ; ΔH = ΔegH
(neutral gaseous atom) (anion)
Evidently, the greater the amount of energy released in the above process, the higher is the electron gain enthalpy of the element.
In other words, it is a measure of the firmness or strength with which an extra electron is bound to it. Like ionization enthalpy, it is measured either in electron volts per atom or kJ per mole.
Depending upon the nature of the element, the process of adding an electron to the atom can be either exothermic or endothermic. For the majority of the elements, energy is released when an electron is added to the atom. Therefore, for such elements, the electron gain enthalpy is negative. For example, the electron gain enthalpy for halogens (i.e., elements of group 17) is highly negative because they can acquire the nearest stable noble gas configuration by accepting an extra electron. In contrast, noble gases have large positive electron gain enthalpies because the extra electron has to be placed in the next higher principal quantum energy level thereby producing highly unstable electronic configuration.
Also Read: Factors Aﬀecting Electron Gain Enthalpy
Successive Electron Gain Enthalpies:
Like second and higher ionization enthalpies, second and higher electron gain enthalpies are also possible. However, after the addition of one electron, the atom becomes negatively charged and the second electron is to be added to a negatively charged ion. But the addition of the second electron is opposed by electrostatic repulsion and hence the energy has to be supplied for the addition of the second electron. Thus the second electron gain enthalpy of an element is positive. For example, when an electron is added to the oxygen atom to form O– ion, energy is released. But when another electron is added to O– ion to form O2- ion, energy is absorbed to overcome the strong electrostatic repulsion between the negatively charged O– ion and the second electron that has been added. Thus,
First electron gain enthalpy: O (g) + e– —-> O– (g) ; ΔegH1 = — 141 kJ mol-1 (Energy is released)
Second electron gain enthalpy: O– (g) + e– ——> O2- (g) ; ΔegH2= + 780 kJ mo1-1 (Energy is absorbed)
The negative of the enthalpy change accompanying the addition of an electron to an isolated gaseous atom is defined as electron affinity (Ae). The electron affinity is said to be positive if energy is released when an isolated gaseous atom accepts an electron and it is assigned negative sign if energy is to be supplied to add an extra electron to the isolated gaseous atom. This is, however, contrary to the thermodynamic convention. Further, since electron affinity is defined at absolute zero, therefore, at any other temperature, heat capacities instead of the electron affinity of the reactants and products should be considered. Therefore, in view of these two reasons, the term electron gain enthalpy is used instead of electron-affinity. The two terms are related to each other as
ΔegH = – Ae – 5/2 RT
Thus numerically electron gain enthalpy is higher than that of electron affinity by 5/2 RT. Since the value of 5/2 RT at 298 is just 2.477 kJ mo1-1, therefore, this small difference is often ignored and the two terms are used indistinguishably with the only difference that electron gain enthalpy is just the negative of electron affinity.