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

Jun 7, 2016

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:

$M \left(g\right) + {e}^{-} \rightarrow {M}^{-} \left(g\right)$

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.