This page deals with the electron affinity as a property of isolated atoms or molecules (i.e. in the gas phase). Solid state electron affinities are not listed here.
Contents
Elements
Electron affinity can be defined in two equivalent ways. First, as the energy that is released by adding an electron to an isolated atom (gas phase). (The energy -or electron affinity- is a scalar quantity and the direction of that energy -released- defines a reaction for which the change in energy ΔE is a negative quantity). The electron affinity is also defined in the case of electron capture as E(initial) – E(final) in order to maintain the positive value. The reverse definition is that the electron affinity is the energy required to remove an electron from a gaseous anion (still a positive quantity, but in which the change in energy ΔE is also a positive quantity). Either convention can be used in practice, but must be consistent in according a scalar, i.e. positive number to the electron affinity.
Negative electron affinities can be used in those cases where electron capture requires energy, i.e. when capture can occur only if the impinging electron has a kinetic energy large enough to excite a resonance of the atom-plus-electron system. Conversely electron removal from the anion formed in this way releases energy, which is carried out by the freed electron as kinetic energy. Negative ions formed in these cases are always unstable. They may have lifetimes of the order of microseconds to milliseconds, but they invariably autodetach after some time. The listed value in the table corresponds to a selected low-lying metastable state, which may or may not be the lowest energy resonance. For example, in He− there is a metastable state with 0.359 ms lifetime at 19.7 eV above the ground state of He, however there is also a lower energy resonance at 19.4 eV that only has a 10−13 s lifetime.
Molecules
The electron affinities Eea of some molecules are given in the table below, from the lightest to the heaviest. Many more have been listed by Rienstra-Kiracofe et al. (2002). The electron affinities of the radicals OH and SH are the most precisely known of all molecular electron affinities.