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

Question

Olivia Anton · Accepted Answer

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 refers to the energy linked to the subsequent 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. Since the nuclear charge is more shielded by the intervening electrons as one descends a Group of the Periodic Table, nuclear attraction to the valence electron should also reasonably decrease, making it LESS favorable. Therefore, electron affinity should increase across a Period but decrease down a Group, which allows us to rationalize the trend using basic electrostatics. But since you're a chemist, you should check the information in the table (and elsewhere) and be assured that I'm not talking about pork pies! Okay, so why are the halogens the atoms with the highest electron affinity?

Peyton Adams · Answer

undefined The trend of increasing electron affinity from left to right is due to increasing effective nuclear charge. The trend of decreasing electron affinity from top to bottom is attributed to increasing atomic size and shielding effects.