Explanation:

* Electron gain enthalpy is the change in energy that occurs when an electron is added to a neutral atom in the gaseous state to form a negative ion.

* Size: Fluorine is smaller in size than chlorine. As a result, the added electron in fluorine experiences a stronger attraction from the nucleus, leading to a greater release of energy.

* Inter-electronic repulsions: In fluorine, the incoming electron is added to the 2p orbital, which is already half-filled. This results in greater inter-electronic repulsions, making it less favorable to add another electron. In chlorine, the added electron goes into a 3p orbital, which is less crowded, leading to fewer repulsions.

* Effective nuclear charge: Although fluorine has a higher nuclear charge, the electrons in the 2p orbitals of fluorine experience a stronger shielding effect from the inner electrons compared to chlorine. This reduces the effective nuclear charge experienced by the added electron in fluorine.

Therefore, despite fluorine being more electronegative, chlorine has a higher electron gain enthalpy due to its larger size, lower inter-electronic repulsions, and a slightly higher effective nuclear charge experienced by the added electron.

Note: The electron gain enthalpy of fluorine is still highly exothermic, but it is less exothermic than that of chlorine.