Isotope Effect

the variation in the properties of the isotopes of a given element or in the properties of compounds differing in isotopic composition, which is caused by the difference in their atomic masses. The differing properties of isotopes determined not by mass but by other characteristics of the atomic nucleus (as manifested in, for example, radioactive decay) are usually not classified as isotope effects.

The difference in the masses of isotopes is responsible for the difference in the masses of the molecules, their moments of inertia, and the strength of the corresponding chemical bonds. This leads both to nonuniform distribution of the isotopes among chemical compounds on reaching equilibrium of isotopic exchange (thermodynamic isotope effects) and to differing rates of a given chemical reaction that transpires with the participation of various isotopic forms of the reacting compounds (kinetic isotope effects). The higher the atomic number of the element the smaller the relative difference in the masses of isotopes. In isotopes of hydrogen it is 100 percent for deuterium D (²H) and 200 percent for tritium T (³H) in comparison with protium H (’H). Therefore, for hydrogen and helium the isotope effects are expressed more strongly. Among them in particular are the isotopic shift and the effects observed on transition to a superconducting state or to a state of superfluidity.

The difference in the masses of the isotopes of a given element is responsible for their differing properties in the isotopic forms of a chemical compound containing the element (such as density, the index of refraction, viscosity, and diffusion coefficient). As a result of isotope effects such thermodynamic properties as heat capacity, thermal conductivity, evaporation heat, heat of fusion, and saturated vapor pressure at a given temperature, as well as the vibrational frequencies of atoms in molecules and crystal lattices, also change.

The use of isotopes as isotope tracers (tagged atoms) is based on the concept of the identical nature of the physical and chemical properties of the isotopes of a given element. As experience shows, this simplifying assumption is close to reality for many isotopes, and the magnitude of isotope effects (both kinetic and thermodynamic) does not exceed the errors of a chemical experiment. However, for light elements the differences in the chemical properties of isotopes may be significant. This must be taken into account when isotopes of light elements, especially the isotopes of hydrogen—deuterium and tritium—are used as tagged atoms. Isotope effects underlie nearly all known laboratory and industrial methods of isotope separation.