The electron affinity (EA) is the energy required/released to add one electron to an atom. For example:
Cl (g) + e- ® Cl- (g) [D E = - 349 kJ/mol]
Note that electron affinity is the opposite process to ionization energy, and keep in mind our thermodynamic sign convention:
D E < 0 Favorable (energy is released, products more stable than reactants)
Periodic Trends in Electron Affinity
1. The electron affinity (generally) increases as you move left to right across the periodic table, from the alkali metals to the halogens.
2. Unlike ionization energy and atomic radii relatively small changes in the electron affinity are observed as you move down a group.
Keep in mind that the electron affinity (just like IE) is a measure of how stable the products are with respect to the reactants. If the products are much more stable, a large amount of energy will be released during the process and EA will be a large negative number. At the other extreme, if the reactants are much more stable than the products, then it becomes very difficult to add an electron and the EA will be positive.
There are enough exceptions to the periodic trends in electron affinity that it is worthwhile to consider electron affinity of specific groups in the periodic table.
Halogens (group 7A, F to At) ® Most negative EA values, addition of an e- leads to noble gas configuration, very favorable.
Group 5A (N to Bi) ® ½ filled shell discourages addition of an electron, EA values less negative than neighbors (groups 4A & 6A).
Alkaline Earths (group 2A, Be to Ba) ® Filled s subshell discourages addition of an electron, EA values nearly zero.
Noble Gases (Group 8A, He to Rn) ® Completely filled shell strongly discourages addition of an electron, EA values are positive.