Electron affinity

The electron affinity (E_ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.

X(g) + e⁻ → X⁻(g) + energy

This differs by sign from the energy change of electron capture ionization.^[1] The electron affinity is positive when energy is released on electron capture.

In solid state physics, the electron affinity for a surface is defined somewhat differently (see below).