The electron affinity (EA) is the energy required/released to add one electron to an atom.
For example:
a) Cl to Cl-
Cl + e- → Cl-
ΔE = - 349 kJ/mol
EA values can be found in tables on the web or in your textbook. EA (generally) increases as you move left to right across the periodic table, from the alkali metals to the halogens. If the products are more stable, a large amount of energy will be released during the process and EA will be a large negative number.