23.1.2—Terms: first electron affinity, EA

First electron affinity is the enthalpy change for X(g) + e⁻ → X⁻(g). It is usually exothermic when the incoming electron is attracted to the nucleus, but the numerical sign depends on the convention used.

Across a period, nuclear attraction generally increases; down a group, distance and shielding increase. Filled or half-filled subshell effects can create small deviations, so explain trends rather than drawing a perfect line.

Halogens have strongly favourable first electron affinities because gaining one electron completes the p subshell. Group 16 values differ from Group 17 because electron pairing and nuclear attraction compete.

Electron affinity is not ionisation energy in reverse, and the second electron affinity is a different process involving an anion.