How would you explain why the trend of electron affinity is increasing from left to right and decreasing from up to down?
Electron affinity is the energy associated with the following reaction:
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,
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?