Chlorine gas - why Chlorine has Highest electron gain enthalpy ?

Electron gain enthalpy is the enthalpy Change take place when an electron is added to isolated gaseous atom .

Electron gain enthalpy of halogens are highly negative because this process is exothermic in nature and large amount of heat is released .

Fluorine is most electronegative element in the periodic table still it's  electron gain enthalpy is less than  Chlorine .

Why Chlorine has High electron gain enthalpy than Flourine ?

Size of fluorine is very small . So when an extra electron tries to attract toward this atom , their is large electron - electron repulsion take place between the outermost electrons and incoming electron . It becomes difficult for incoming electron to add up in the outermost shell .

On the other hand , Chlorine is larger in size than flourine and when extra electron come in contact with this atom , Chlorine immediately tries to attract toward itself . Due to larger size , their is less electron - electron repulsion and it becomes easier for an electron to enter the outermost orbit. 

Therefore , Chlorine has more electron gain enthalpy than flourine