Elements
- Note on negative electron affinities
- Those elements listed with negative electron affinity are known to only have unstable negative ions. In some cases these negative ions can be prepared (and may have lifetimes on the order of microseconds to milliseconds) but they invariably autoionize after some time. The listed value in the table corresponds to a selected low-lying metastable state. This may or may not be the lowest energy resonance; for example in Heâˆ' 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 10âˆ'13 s lifetime.
Electron affinity can be defined in two equivalent ways. First, as the energy [that is] released by adding an electron to a gaseous atom. (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 Î"E is a negative quantity). The electron affinity is also defined in the case of electron capture as E(initial) â€" 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 Î"E 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. A negative sign appearing with a value for electron affinity indicates a change in direction, either that electron capture requires energy or removal releases energy.
Molecules
Bibliography
- CRC Handbook of Chemistry and Physics (62nd Edn. (1981); Weast, Robert C. (ed)). Boca Raton, FL: CRC Press. "Section E, General Physical Constants; Electron Affinities".
- Janousek, B. K.; Brauman, J. I. (1979). in Gas Phase Ion Chemistry (Bowers, M. T. (Ed.)), Vol. 2. New York: Academic Press. p. 53.