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.