Like the second and higher ionisation energies , the second and higher electron affinities are also possible. After addition of one electron, the atom becomes negatively charged and the second electron is added to a negatively charged ion. The addition of second electron is opposed by the coulombic force of repulsion and energy has to be supplied for the addition of the second electron. If an atom has spontaneous tendency, i.e., a positive tendency , to gain electron, then conventionally , its electron gain enthalpy is said to be negative and if the atom is reluctant to gain an electron, i.e., it has a negative tendency to gain an electron and is forced to accept it, its electron gain enthalpy is positive. Thus in the case of oxygen, the first electron gain enthalpy is negative since 141 kJ is released when one mole of oxygen atoms get converted to O- ions. In other words oxygen atom has positive tendency to accept electron. However, the second electron gain enthalpy is positive since 770 kJ of energy has to be supplied to convert 1 mol of O- ions to O2- ions.
Similarly in the case of sulphur, while first electron gain enthalpy is negative since 200 kJ of energy is released when 1 mole of S atoms get converted to S- ions and second electron gain enthalpy is positive since 590 kJ of energy has to be supplied to convert 1 mole of S- ions to S2- ions.
Thermodynamically , the energy released is given a negative sign and energy absorbed is given a positive sign. Accordingly, when a species has a positive electron affinity , DH , accompanying the addition of an electron to the species, isnegative and if it has a negative electron affinity, DH is positive. Thus for the reaction Cl + e ® Cl- , while electron affinity is positive , while electron gain enthalpy is negative. In various calculations involving DH, the value of electron affinity of chlorine would be taken as -349 kJmol-1and not +349 kJmol-1.