Explain the periodicity of electron gain enthalpy in a group.

Electron gain enthalpy is the total enthalpy released when an electron is added to the outermost shell of an isolated gaseous atom.

As one goes down a group, the atomic size increases, which means the distance between the nucleus and the outermost electron also increases. At the same time, there is no significant change in the effective nuclear charge down the group. Hence, the ability of nucleus to hold the new incoming electron effectively, also decreases.

As a result, the value of electron gain enthalpy generally decreases down the group.