Another reason for the electron affinity group trend is increased in atomic radius. The reason for this is nuclear charge which causes increase in electron affinities. It takes more energy to add electrons the farther down the group is. The group trends for electron affinity is from top to bottom the electron affinity decreases. This property makes the halogens ready and happy to bond with other atoms to fill the last space in order to achieve noble gas configuration. The halogens are a group of elements who share the property of needing a single electron to become stable. A prime example of electron affinity is Group 17: halogens. The electron affinity trend across periods from left to right is from left to right, the electron affinity increases in electron affinity value although the numbers become less and less(negative values).

Electron affinity also experiences trends or patterns in both periods and groups. Exothermic reactions produce negative energy, which is denoted by a negative sign, whereas endothermic reactions produce positive energy, which is denoted by a positive sign. If they do exist the values for the electron affinities are all positive. Electron affinity is defined as the change in energy (in kJ/mol) of a neutral atom (in the gaseous phase) when an electron is added to form a negative ion. Electron affinity is rarely applied to second or third electron affinity levels because the elements are so unstable, and thus unable to survive long in nature.

Electron affinity is the energy change that occurs when an electron is added to a neutral atom.