Why is electron gain enthalpy of …

Electron gain enthalpy is defined as the enthalpy change associated with an isolated gaseous atom  when it gains an electron to form its corresponding anion.

X_(g)  +    e^- --------->   X^- _(g)   

General trend for variation of electron gain enthalpy  down the group.

On moving down the group the atomic size as well as nuclear charge increases. But the effect of increase in atomic size is much more pronounced than that of nuclear charge. Thus , the additional electron feels less attraction  with the result electron gain enthalpy becomes less  negative on going down the group.

However in case of halogens , there happens to be an exception  as  given below.

F =  -328 kj /mol

Cl  =  -349 kj /mol

Br  = -325  kj /mol

  I  =  -295 kj / mol 

Electron gain enthalpy  of  F  is less negative than Cl because of its smaller size. The additional electron in case of F will be added to 2p orbital , while in case of Cl it will go in the 3p orbital. as 2p orbital occupies a smaller region of space than that of 3p orbital, it will be more compact  , due to which electron - electron repulsion would be more. Therefore, the incoming electron would be accepted  with much less ease in case of  F.  Hence , its electron gain enthalpy is less negative than Cl.