Question

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

Answer 1

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

Explanation:

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.