Exothermic electron affinity refers to the process where energy is released when an electron is added to an atom.

A prime example of exothermic electron affinity is the addition of an electron to a chlorine atom. Chlorine possesses seven valence electrons and requires one additional electron to complete its octet. When an electron is added to a chlorine atom, energy is released in the form of heat. This energy release occurs because the added electron is attracted to the positively charged nucleus of the atom; as the electron approaches, the potential energy of the system decreases. The energy released during this process is quantified as the electron affinity of chlorine, which is approximately $-349 \, \text{kJ/mol}$.

In contrast, endothermic electron affinity occurs when energy is absorbed during the addition of an electron to an atom. An illustrative example of this is the addition of an electron to a helium atom. Helium has two valence electrons and does not require any additional electrons to complete its electron configuration. When an electron is added to a helium atom, energy must be supplied to overcome the repulsion between the negatively charged incoming electron and the existing electrons in the atom. The energy absorbed in this case is represented by the electron affinity of helium, which is approximately $+20 \, \text{kJ/mol}$.

In summary, exothermic electron affinity involves the release of energy when an electron is added to an atom, while endothermic electron affinity requires energy input for the same process. The electron affinity of an atom is a measure of the energy change that occurs when an electron is added, and it can take on either a positive or negative value.