Amount of energy released when an electron is added to an isolated gaseous neutral atom is called electron gain enthalpy. It is denoted ∆egH
A (g) + e– ➝ A– (g) ; ∆H = ∆egH
Electron gain enthalpy is measured in electron volts per atom or kJ per mole. Higher the energy released in the process of taking an extra electron, the higher will be electron gain enthalpy. Higher the value of electron gain enthalpy of an atom, the more is the tendency to change into anion.
Factors Affecting Electron Gain Enthalpy :
(i) Effective Nuclear Charge : Electron gain enthalpy is directly proportional to nuclear charge. With increase in effective nuclear charge, electron gain enthalpy increases.
(ii) Atomic size: Electron gain enthalpy is inversely proportional to atomic size, with increase in atopiic size, value of electron gain enthalpy decreases.
(iii) Shielding Effect : Electron-gain enthalpy is inversely proportional to the shielding effect. As shielding effect increases, effective nuclear charge decreases and electron gain enthalpy decreases.
(iv) Half Filled and Fully Filled orbitals: Half-filled and fully filled orbitals are stable. Therefore, their electron gain enthalpy is low.
Periodicity:
(i) Electron gain enthalpy increases from left to right across a period due to decrease in atomic size and increase in ionisation enthalpy.
(ii) Electron enthalpy decreases from top to bottom in going down the group due to increased atomic size.
