- Electron affinity
The electron affinity, "E"ea, of an
atomor moleculeis 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 ("E"initial − "E"final) when an electron is attached to a neutral atom or molecule. It should be noted that the sign conventionfor "E"ea is the opposite to most thermodynamicquantities: 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 "E"ea, meaning they would repel electrons.Fact|date=May 2008 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 "E"ea.
Chlorinemost 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,
nonmetalshave more positive "E"ea 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.
"E"ea 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 "E"ea 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
"E"ea is not limited to the elements but also applies to molecules. For instance the electron affinity for
benzeneis negative, as is that of naphthalene, while those of anthracene, phenanthreneand pyreneare positive. " In silico" experiments show that the electron affinity of hexacyanobenzenesurpasses 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 [http://dx.doi.org/10.1039/b515843e Abstract] ] .
*Tro, Nivaldo J. (2008). "Chemistry: A Molecular Approach" (2nd Edn.). New Jersey:
Pearson Prentice Hall. ISBN 0-13-100065-9. pp. 348–349.
* [http://www.iupac.org/goldbook/E01977.pdf Electron affinity] , definition from the
IUPAC Gold Book
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