In chemistry, electron affinity is the amount of energy absorbed when an electron is added to a neutral isolated gaseous atom to form a gaseous ion with a 1- charge. It has a negative value if energy is released.

Most elements have a negative electron affinity. This means they do not require energy to gain an electron; instead, they release energy. Atoms more attracted to extra electrons have a more negative electron affinity. Chlorine most strongly attracts extra electrons; radon most weakly attracts an extra electron.

Although electron affinities vary in an chaotic manner across the table, some patterns emerge. Generally, nonmetals have more negative electron affinities than metals. However, the noble gases are an exception: they have positive electron affinities.

Electron affinity is influenced by the octet rule. Group 1 elements (hydrogen, lithium, sodium, potassium, rubidium, caesium, and francium) tend to gain an electron and form -1 anions. The noble gases have a full octet, and thus adding an extra electron requires large amounts of energy, but it is possible.