The factors affecting electron gain enthalpy and the trend in its variation in the periodic table are:

1. Atomic size – As the distance between the nucleus and the outermost shell rises, the tendency of the atom to gain electron diminishes, and the electron gain enthalpy become less negative.
2. Effective nuclear charge – The effective nuclear charge grows as we move from left to right in a period and drops as we move down the group, resulting in the attraction of electrons from the outermost shell.
3. Electronic configuration – The ability of an element to gain electrons is determined by its stability. Elements with fully or partially filled stable orbitals have a low tendency to gain electrons, and so have a low electron gain enthalpy. The electron gain enthalpy grows more negative across periods. The electron gain enthalpy decreases as one moves down the group.