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Electron gain enthalpy is the energy released when an electron is added to an isolated gaseous atom.

Its value is negative and the ease with which an atom accepts electrons its electron gain enthalpy becomes more negative that is, its value increases.

With an increase in nuclear charge, the electron gain enthalpy also increases.

Due to the extra stability of nitrogen (it has half-filled valence shell \([ He ] \,1 s ^2 \,2 p ^3\), which provides extra stability to it), it is very difficult to add an electron to it.

Therefore, it has almost zero electron gain enthalpy.

In general, on moving down the group, the electron gain enthalpy decreases due to an increase in the atomic size, but on moving from \(F\) to \(Cl\) the electron gain enthalpy increases.

Due to the very small size of \(F\) atom and absence of \(3^{\text {rd }}\) shell, the incoming electron experiences repulsion due to the valence electrons and its electron gain enthalpy is less than \(Cl\). After \(Cl\) the electron gain enthalpy follows the general order and decreases on moving down to \(Br\) and \(I\).