Electron affinities

The first electron affinity (EA₁) is minus the internal energy change (equation 1.1) for the gain of an electron by a gaseous atom (equation 1.2). The second electron affinity of atom Y is defined for process 1.3. Each reaction occurs in the gas phase.

   As we saw for ionization energies, it is convenient to define an enthalpy change, Δ_EAH, associated with each of the reactions 1. 1 and 1. 2. We approximate Δ_EAH(298K) to Δ_EAU(0K). Selected values of these enthalpy changes are given in Table 1.1.

Table 1.1 Approximate enthalpy changes Δ_EAH(298 K) associated with the attachment of an electron to an atom or anion.

  The attachment of an electron to an atom is usually exothermic. Two electrostatic forces oppose one another: the repulsion between the valence shell electrons and the additional electron, and the attraction between the nucleus and the incoming electron. In contrast, repulsive interactions are dominant when an electron is added to an anion and the process is endothermic (Table 1.1).