INTERVIEWER: Electron affinity is the energy change that occurs when an electron is added to a gaseous atom or ion. For example, when an electron is added to a neutral gaseous chlorine atom, 349 kilojoules per mole is released. In the case of chlorine and the other halogen elements, addition of an electron to the neutral atom is an exothermic process. Addition of an electron to the neutral oxygen atom is also an exothermic process. However, addition of a second electron to form the oxide ion is an endothermic process. This means that work must be done to force an electron onto the singly charged ion to form the oxide ion, O2 minus.