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The electron affinity, E_ea, of an atom or molecule is the energy required to detach an electron from a singly charged negative ion, that is the energy change for the process

X → X + e

An equivalent definition is the energy released (E_initial − E_final) when an electron is attached to a neutral atom or molecule. It should be noted that the sign convention for E_ea is the opposite to most thermodynamic quantities: a positive electron affinity indicates that energy is released on going from atom to anion.

All elements have a positive electron affinity, but older texts mistakenly report that some elements such as inert gases have negative E_ea, meaning they would repel electrons. This is not recognised by modern chemists. Atoms whose anions are relatively more stable than neutral atoms have a more negative E_ea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. E_ea of noble gases are close to 0.

Although E_ea vary in a chaotic manner across the table, some patterns emerge. Generally, nonmetals have more positive E_ea than metals.

Values for the elements

The following data are quoted in kJ/mole. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds.

Periodic trends

E_ea increases across a period (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 group (family) (because of a large increase in radius and number of electrons that decrease the stability of the atom, repulsing each other).

Molecular electron affinities

E_ea is not limited to the elements but also applies to molecules. For instance the electron affinity for benzene is positive, that of naphthalene also positive and that of anthracene,phenanthrene and pyrene is negative. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene .

See also

• Koopmans' theorem
• One-electron reduction
• ionization potential
• electronegativity
• Valence Electrons

References

1. 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

External links

• Electron affinity, definition from the IUPAC Gold Book
• Article about Electron Affinity

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