The electron affinity
, of an atom
is the energy required to detach an electron from a singly charged negative ion
, i.e., the energy change for the process
- X- → X + e−
An equivalent definition is the energy released (Einitial
) when an electron is attached to a neutral atom or molecule. It should be noted that the sign convention
is the opposite to most thermodynamic
quantities: a positive electron affinity indicates that energy is released
on going from atom to anion
All elements whose EA have been measured using modern methods have a positive electron affinity, but older texts mistakenly report that some elements such as alkaline earth metals have negative Eea, meaning they would repel electrons. This is not recognized by modern chemists. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative EAs. Atoms whose anions are relatively more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. Eea of noble gases are close to 0.
Although Eea vary in a chaotic manner across the table, some patterns emerge. Generally, nonmetals have more positive Eea than metals.
Values for the elements
The following data are quoted in kJ/mol
. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds. Elements marked with a dotted box are synthetically made elements—elements not found naturally in the environment.
generally increases across a period (row) in the periodic table. This is caused by the filling of the valence shell of the atom; a group 7A atom releases more energy than a group 1A atom on gaining an electron because it obtains a filled valence shell.
A trend of decreasing Eea going down the groups in the periodic table would be expected. The additional electron will be entering an orbital farther away from the nucleus, and thus would experience a lesser effective nuclear charge. However, a clear counterexample to this trend can be found in group 2A, and this trend only applies to group 1A atoms.
Molecular electron affinities
is not limited to the elements but also applies to molecules. For instance the electron affinity for benzene
is negative, as is that of naphthalene
, while those of anthracene
are positive. In silico
experiments show that the electron affinity of hexacyanobenzene
surpasses that of fullerene
- Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Edn.). New Jersey: Pearson Prentice Hall. ISBN 0-13-100065-9. pp. 348–349.