How would you explain why the trend of electron affinity is increasing from left to right and decreasing from up to down?

1 Answer
Nov 26, 2015

Electron affinity is defined as the enthalpy change when a gaseous ATOM binds to an electron, to form a gaseous anion. This should reasonably follow a Periodic trend.


Electron affinity is the energy associated with the following reaction:

#E(g) + e^(-) rarr E^(-)(g)# #DeltaE# #=# electron affinity.

It is reasonable to assume that as we go from left to right across a Period, this process should become MORE favourable. Why? As we go from left to right, the nuclear charge, #Z#, increases sequentially, while electrons are added to the same shell.

It should reasonably become LESS favourable as we descend a Group of the Periodic Table inasmuch as the nuclear charge becomes more shielded by the intervening electrons, and nuclear attraction to the valence electron should decrease. Thus electron affinity should increase across a Period, but decrease down a Group. Thus we can rationalize the trend on the basis of simple electrostatics.

As a chemist, however, you should consult the data in the table provided (and elsewhere), and satisfy yourself that I am not telling pork pies! So why are the halogens the atoms with the greatest electron affinity?