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Electron affinity
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<p>The electron affinity (<i>E</i>ea) of an <a href="/facts/Atom/FoQ4kGN5">atom</a> or <a href="/facts/Molecule/N9ljSfFq">molecule</a> 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. </p> X(g) + e− → X−(g) + energy <p>This differs by sign from the energy change of <a href="/facts/Electron_capture_ionization/qtQGR4Rx">electron capture ionization</a>. The electron affinity is positive when energy is released on electron capture. </p><p>In <a href="/facts/Solid_state_physics/Cmc6hvyR">solid state physics</a>, the electron affinity for a surface is defined somewhat differently (see below). </p>