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 a 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 trends:

• Electron affinity is influenced by the octet rule. Group 17 elements (fluorine, chlorine, bromine, iodine, and astatine) tend to gain an electron and form -1 anions. The noble gases in group 18 already have a full octet, and thus adding an extra electron requires large amounts of energy, but it is possible.
• Group 2 elements starting with beryllium and group 12 elements starting with zinc also have positive electron affinity values because these elements have a filled s subshell or d subshell.
• The elements in group 15 have low electron affinities and that of nitrogen is even positive. The reason is that stabilization is even gained from half-filled subshells.
• The electron affinities increase across a row (since the radius slightly decreases, because of the increased attraction from the nucleus, and the number of electrons in the top shell increases, helping the atom reach maximum stability) in the periodic table and decrease going down a family (because of a large increase in radius and number of electron that decrease the stability of the atom, repulsing each other).

Electron affinities are not limited to the elements but also apply to molecules. For instance the electron affinity for benzene is positive, that of naphthalene near zero and that of anthracene positive. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene Remarkable electron accepting properties of the simplest benzenoid cyanocarbons: hexacyanobenzene, octacyanonaphthalene and decacyanoanthracene Xiuhui Zhang, Qianshu Li, Justin B. Ingels, Andrew C. Simmonett, Steven E. Wheeler, Yaoming Xie, R. Bruce King, Henry F. Schaefer III and F. Albert Cotton Chemical Communications, 2006, 758 - 760 Abstract.

See also

• Koopmans' theorem
• One-electron reduction

References