The enthalpy change accompanying the addition of an electron to a neutral gaseous atom is called electron gain enthalpy.
- Electron gain enthalpy becomes more negative along the period. Reason: This is because the effective nuclear charge increases from left to right and it will be easier to add an electron to a smaller atom.
- Electron gain enthalpy becomes less negative down the group. Reason: This is because size of the atom increases and the added electron will be farther from the nucleus.