Electron affinity

Measurement and conventions

Values are determined experimentally by methods such as beam experiments, spectroscopy and thermochemical cycles, and can also be estimated by quantum chemical calculations. Because isolated-gas measurements avoid solvent and lattice effects, gas-phase electron affinities are the standard reference. When an electron is added to an atom that already carries a negative charge, the second electron affinity is usually endothermic because of electrostatic repulsion.

Periodic trends and controlling factors

Across a period from left to right, electron affinity generally becomes more exothermic (atoms more readily accept electrons) due to increasing effective nuclear charge and smaller radii. Down a group the tendency often decreases because added electrons are placed farther from the nucleus and experience greater shielding. Several factors influence the value:

• Nuclear charge and effective charge felt by the added electron
• Electron shielding by inner shells
• Atomic radius and orbital type receiving the electron (s, p, d)
• Electron-electron repulsion and subshell electronic configuration

For a concise discussion of trends, consult atomic properties references and periodic trend summaries.

History and development

Interest in electron affinities grew alongside advances in spectroscopy and mass spectrometry in the 20th century, which allowed direct study of negative ions and their formation energies. Improvements in experimental techniques and computational methods progressively refined reported values and clarified exceptions to simple trends. Historical overviews and measurement techniques are reviewed in specialized texts and articles on experimental methods.

Importance, examples and distinctions

Electron affinity helps predict chemical behavior: elements with large exothermic affinities tend to form stable anions and participate in electron-transfer reactions. Halogens are classic examples of high electron affinity; noble gases have very low or effectively unfavorable affinities. The concept is important in fields from inorganic chemistry and materials science to atmospheric chemistry and semiconductor doping. For applications and further examples see applied discussions.