Electron affinity

(atomic physics) The work needed in removing an electron from a negative ion, thus restoring the neutrality of an atom or molecule.

The amount of energy released when an electron at rest is captured by a species M, producing the negative ion M⁻. The electron affinity of a species M can also be thought of as the ionization potential of the negative ion M⁻. Stated in terms of a chemical equation, the electron affinity of a species M is equal to the exothermocity of the reaction e + M → M⁻, where the negative ion M⁻ is left in its lowest electronic, vibrational, and rotational state. See also Ionization potential.

If the electron affinity of M is negative, the M⁻ ion is unstable with respect to decomposition into M + e. Most atoms have positive electron affinities, even though there is no net Coulomb attraction between the electron and the atom until the electron is close enough to be “a part of the atom.” The simple rules of chemical valency provide a qualitative guide to the magnitude of electron affinities. Thus the noble gases, which have a filled outer electronic shell and are chemically inert, are not capable of binding an additional electron to form a negative ion. The largest electron affinities are possessed by the halogens, atoms which require only one additional electron to fill the valence shell. See also Valence.

The major exception to this concept is that multiply charged negative ions—for example, O²⁻, one of many multiply charged negative ions which are stable in solution—are not stable in the gas phase. The ability to place more than one additional electron in the valence shell of a neutral atom or molecule appears to come from the medium: the solvent shell surrounding the ion in liquid solutions and the amorphous or crystalline region surrounding the ion in solids.