The molar enthalpy change accompanying the addition of an electron to an atom (or ion) is known as electron gain enthalpy.
Generally in increases on moving from left to right in a period and in a group it decreases as the size increases.
Exception: Because of the small size of F, electron-electron repulsion present in its relatively compact 2p-subshell, do not easily allow the addition of an extra electron. On the other hand, Cl because of its comparatively bigger size than F, allows the addition of an extra electron more easily.
$\mathop O\limits_{ - 1.48} \lt \mathop S\limits_{ - 2.0} \lt \mathop F\limits_{ - 3.6} \lt \mathop {Cl}\limits_{ - 3.8}$