Why is the trend of electron affinity increasing from left to right and decreasing from up to down?

Question

Jeremiah Adams · Accepted Answer

Two factors are important here: (i) nuclear charge; and (ii) shielding by other electrons.

Electron affinity measures the enthalpy associated with the formation of 1 mole of gaseous anions, from 1 mole of gaseous elements, and 1 mole of electrons: #M(g)+e^(-) rarr M^(-)(g)# Two factors determine the thermodynamics of ths process: (i) nuclear charge; and (ii) shielding by other electrons. As #Z# increases across the Period, nuclear charge wins over electronic shielding, and electron affinities should increase across the Period. A full electronic shell, however, should shield the nuclear charge, and the electron affinities of the Noble Gases should be least exothermic. 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.

Xavier Avery · Answer

undefined The trend of electron affinity increases from left to right across a period because as you move across a period, the effective nuclear charge increases, leading to stronger attraction between the nucleus and the added electron. However, electron affinity decreases from top to bottom within a group due to increased atomic size, resulting in weaker attraction between the nucleus and the added electron.