Electron affinity: the definition, first and second, and the sign
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
The first electron affinity is the enthalpy change when one electron is added to each atom in one mole of gaseous atoms, forming one mole of gaseous 1- ions. The equation is X(g) + e- becomes X-(g).
Words that must appear
- One electron added to each atom
- One mole of gaseous atoms
The sign
The first electron affinity of most non-metals is exothermic, because the incoming electron is attracted to the nucleus. The second is endothermic, because the electron is being added to an ion that is already negative.
Do not confuse with
Ionisation energy, which removes an electron.
Common questions
What is the first electron affinity?
The enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
Is electron affinity exothermic or endothermic?
The first is usually exothermic. The second is endothermic.
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