Electron gain enthalpy is defined as the amount of energy released when an electron is added to an isolated gaseous atom.
Electron gain enthalpy: On moving from left to right, atomic size decreases and effective nuclear charge increases. Thus, electron gain enthalpy becomes more negative. On moving down the group, atomic size increases. Thus, electron gain enthalpy becomes less negative.
| Element | ΔegH(kJ/mol) |
| F | –333 |
| S | –200 |
| Br | –325 |
| Ar | +96 |