This page deals with the
electron affinity
The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.
::X(g) + e− → X−(g) + energy
This differs by si ...
as a property of isolated
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s or
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s (i.e. in the
gas phase).
Solid state electron affinities are not listed here.
Elements
Electron affinity can be defined in two equivalent ways. First, as the energy that is released by adding an electron to an isolated gaseous atom. The second (reverse) definition is that electron affinity is the energy required to remove an electron from a singly charged gaseous negative ion. The latter can be regarded as the ionization energy of the −1 ion or the ''zeroth'' ionization energy. Either convention can be used.
Negative electron affinities can be used in those cases where electron capture requires energy, i.e. when capture can occur only if the impinging electron has a kinetic energy large enough to excite a
resonance
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
of the atom-plus-electron system. Conversely electron removal from the anion formed in this way releases energy, which is carried out by the freed electron as kinetic energy. Negative ions formed in these cases are always unstable. They may have lifetimes of the order of microseconds to milliseconds, and invariably
autodetach after some time.
† A quantum offset of the velocity imaging-based measurements was revealed in 2025,
[ which could make a revision of all electron affinities marked with a dagger † necessary. The value of the downward correction to be applied is determined by the intensity of the electric field that was used in the experiment, which was not published with the original measurements, but can be estimated to be of the order of −20 μeV.
]
Molecules
The electron affinities ''E''ea of some molecules are given in the table below, from the lightest to the heaviest. Many more have been listed by . The electron affinities of the radicals OH and SH are the most precisely known of all molecular electron affinities.
Second and third electron affinity
Bibliography
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* Updated values can be found in th
NIST chemistry webbook
for around three dozen elements and close to 400 compounds.
Specific molecules
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* [According to ]NIST
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
as concern
Boron trifluoride
the Magnetron method, lacking mass analysis, is not considered reliable.
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References
See also
{{Navbox periodic table
Atomic physics
Chemical properties
Chemical element data pages