Electron Gain Enthalpy

Electron Gain Enthalpy

Halogens record the most negative electron gain enthalpies of any elements in their respective periods — unsurprising, since accepting one more electron completes their octet and gives them a noble-gas configuration. As with most other periodic trends, this negative electron gain enthalpy becomes less negative down the group.

There is a well-known exception, though: fluorine's electron gain enthalpy is actually less negative than chlorine's, even though fluorine sits directly above it. The reason lies in fluorine's very small atomic size — its 2p orbitals are so compact that the electrons already present repel one another strongly (interelectronic repulsion), and this repulsion partly offsets the attraction the incoming electron would otherwise feel. …