Electron Affinity
The Electron affinity of a molecule or atom is the energy change when an electron is added to the neutral species to form a negative ion. This property can only be measured in an atom in gaseous state.
X + e− → X−
The electron affinity, Eea, is defined as positive when the resulting ion has a lower energy, i.e. it is an exothermic process that releases energy:
Eea = Einitial − Efinal
Alternately, electron affinity is often described as the amount of energy required to detach an electron from a singly charged negative ion[1], i.e. the energy change for the process
X− → X + e−
A molecule or atom that has a positive electron affinity is often called an electron acceptor and may undergo charge-transfer reactions.
Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative EAs.
Eea 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. Electron affinity follows the trend of electronegativity. Fluorine (F) has a higher electron affinity than oxygen and so on.
The electron affinity of molecules is a complicated function of their electronic structure. For instance the electron affinity for benzene is negative, as is that of naphthalene, while those of anthracene, phenanthrene and pyrene are positive. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene[2].
Electron affinity of Surfaces
The electron affinity measured from a material's surface is a function of the bulk material as well as the surface condition. Often negative electron affinity is desired to obtain efficient cathodes that can supply electrons to the vacuum with little energy loss. The observed electron yield as a function of various parameters such as bias voltage or illumination conditions can be used to describe these structures with band diagrams in which EA is one parameter. For one illustration of the apparent effect of surface termination on electron emission,
X + e− → X−
The electron affinity, Eea, is defined as positive when the resulting ion has a lower energy, i.e. it is an exothermic process that releases energy:
Eea = Einitial − Efinal
Alternately, electron affinity is often described as the amount of energy required to detach an electron from a singly charged negative ion[1], i.e. the energy change for the process
X− → X + e−
A molecule or atom that has a positive electron affinity is often called an electron acceptor and may undergo charge-transfer reactions.
Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative EAs.
Eea 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. Electron affinity follows the trend of electronegativity. Fluorine (F) has a higher electron affinity than oxygen and so on.
The electron affinity of molecules is a complicated function of their electronic structure. For instance the electron affinity for benzene is negative, as is that of naphthalene, while those of anthracene, phenanthrene and pyrene are positive. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene[2].
Electron affinity of Surfaces
The electron affinity measured from a material's surface is a function of the bulk material as well as the surface condition. Often negative electron affinity is desired to obtain efficient cathodes that can supply electrons to the vacuum with little energy loss. The observed electron yield as a function of various parameters such as bias voltage or illumination conditions can be used to describe these structures with band diagrams in which EA is one parameter. For one illustration of the apparent effect of surface termination on electron emission,
- ^ International Union of Pure and Applied Chemistry. "Electron affinity". Compendium of Chemical Terminology Internet edition.
- ^ 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
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