Electron gain enthalpy is the energy change when an electron is added to a neutral gaseous atom. Generally, it becomes more negative (more exothermic) as we move across a period and less negative as we move down a group due to increasing atomic size.
For the halogens (Group 17), the expected trend based on size would be $F > Cl > Br > I$ (in terms of magnitude of energy released). However, Fluorine has a very small atomic size, which leads to significant inter-electronic repulsions when an incoming electron is added to its compact 2p subshell.
Due to the high electron-electron repulsion in the small 2p orbital of Fluorine, the energy released upon adding an electron to Chlorine is greater than that for Fluorine. Thus, the order is $Cl > F > Br > I$.
Final Answer: Cl > F > Br > I
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