Electron gain enthalphy is the amount of energy involved when an isolated gaseous atom accepts an electron to form a monovalent anion
the value of the electron gain enthalpy of halogens are given below:
F : `-328 KJ mol^(-1) Cl - 349 KJ mol^(-1) Br : -325 KJ mol^(-1) I : -295 KJ mol^(-1) Why the negative electron gain enthalpy decreases from chlorine iodine?

Explanation

Electron gain enthalpy becomes less negative from chlorine to iodine because of the increase in size. As atomic size increase, the added electron is less attracted by the nucleus and so the energy released will be less.