Electron Affinity

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electron affinity

[i′lek‚trän ə′fin·əd·ē]
(atomic physics)
The work needed in removing an electron from a negative ion, thus restoring the neutrality of an atom or molecule.

Electron Affinity


the ability of some neutral atoms, molecules, and free radicals to capture additional electrons and thereby become negative ions. For each specific type of particle, this ability is measured by the quantity S, known in English simply as the electron affinity. 5 is equal to the energy difference between the neutral atom or molecule in the ground state and the ground state energy of the negative ion formed after the addition of the electron.

For most atoms, the ability to add an electron results from the atoms’ outer electron shells not being filled (see). Such atoms include H atoms and elements of Group I of the periodic table, which have one outer s electron, and also atoms of groups III, IV, V, VI, and VII, which have incomplete shells. The capture of an additional electron by Fe, Co, and Ni atoms, which in the normal state have two outer electrons, is generally believed to lead to the filling of a free position in the inner 3d shell.

The value of S has been accurately determined for only a few atoms; the data on the S of molecules and radicals are, for the most part, insufficiently reliable. The 5 of atoms can be measured directly, for example, by determining the wavelength of light λ0 corresponding to the threshold of photodetachment of an electron from the negative ion: S = hcλ0, where h is Planck’s constant and c is the speed of light. The values of S for C, O, S, I, and Cl atoms have been established by this method. The use of the surface ionization effect (the vaporization of halogen atoms from the surface of incandescent W) to measure 5 has not yet yielded accurate values of 5. The reason for this failure is that, because of the polycrystalline structure of W, the work function is not the same on different parts of the surface. When two atoms are vaporized from the same surface and become negative ions, the difference in the 5 of the two atoms can be determined with much higher accuracy. Typical values of S for atoms, in electron volts (eV), are as follows: H, 0.754; C, 1.25; 0, 1.46; S, 2.1; F, 3.37; Cl, 3.65; Br, 3.35; and 1,3.08. The values of 5 for molecules and radicals vary over a wide range. In many cases they amount to fractions of an eV. Larger values, however, are also found: NO2, > 3 eV; OH ~2 eV; and CN, <3 eV.

References in periodicals archive ?
Depending on the local electron affinity of the condensed metallic hydrogen, the number of electrons transferred, n, could range from single digits to ~25 [192] in the case of iron.
The governing force in each case would be the electron affinity of metallic hydrogen which may increase with altitude.
The changes in atomic and ionic compositions observed in the solar atmosphere can be accounted for by 1) the varying ability of molecular species to deliver hydrogen and protons to condensed hydrogen structures in the chromosphere as a function of altitude, and 2) to changes in the electron affinity of metallic hydrogen in the corona.
Lightning represents the longest standing example of the power of electron affinity in condensed matter.
The presence of metallic hydrogen in the corona may then promote, through its elevated electron affinity, the creation of highly ionized species.
Such a structure, if endowed with a elevated electron affinity [2], would provide an elegant network for channeling electrons from the outer reaches of the solar atmosphere onto the photospheric surface.
In the corona, highly ionized ions are produced when their parent atoms, or ions, come into contact with metallic hydrogen which possesses an elevated electron affinity.