What is Electron Affinity?

The amount of energy (in kJ) released to form 1 mole of gaseous negative ions by adding 1 mole of electrons to 1 mole of gaseous atoms of an element is known as the electron affinity (EA) of the element.

For example, if M is an element in the periodic table, then its electron affinity can be expressed as follows:

$M(g)+e^-→M^-(g)$

Unit: kJ/mol [Kilojoules per mole]