Why is the trend of electron affinity increasing from left to right and decreasing from up to down?
Two factors are important here: (i) nuclear charge; and (ii) shielding by other electrons.
Two factors determine the thermodynamics of ths process: (i) nuclear charge; and (ii) shielding by other electrons. As
AS the valence shell become larger (as we descend a Group, a column, on the Periodic Table) electron affinities should reasonably decrease on the basis of shielding.