Important Trends in Electron Gain Enthalpy: There are some important features of electron gain enthalpies of elements these are :
Halogens have the highest negative electron gain enthalpies:The electron gain enthalpies of the halogens are highly negative. This is due to the fact that halogens have the general electronic configuration of ns2np5 and have only one electron less than the stable noble gas configuration ns2np6.
Electron gain enthalpy values of noble gases are positive while those of Be, Mg, N and P are almost zero: The electron gain enthalpy values of noble gases are positive. This is because they have stable electronic configuration of ns2np6 and thus they have absolutely no tendency to take an additional electron. This, that the incoming electron enters the next higher principal quantum level and does not feel any attract the nucleus. Thus, energy is required to force the electron in their atoms and therefore, their electron gain enthalpies are positive.
• Electron gain enthalpy of fluorine is unexpectedly less negative than of chlorine: As already explamoin electron gain enthalpy, in general, becomes less negative from top to bottom in a group, However, it is observed that F-atom has unexpectedly less negative electron gain enthalpy than Cl-atom. This is due to the very small seze of F-atom. As a consequence of small size, there are strong inter electronic repulsions in the relatively compact 2p-subshell of fluorine and thus the incoming electron does not feel much attraction.