Electron Gain Enthalpy

Electron gain enthalpy in Group 16 does not fall into a single simple trend. Oxygen, despite being the most electronegative element in the group, has a less negative electron gain enthalpy than sulphur — a consequence of oxygen's very compact atomic size, which means an incoming electron experiences significant repulsion from the electrons already crowded into its small $2p$ subshell. From sulphur onward, however, the more familiar group trend reasserts itself: the electron gain enthalpy becomes progressively less negative all the wa …