Isotopes are atoms whose nuclei contain the same number of protons but a different number of neutrons. There are about 300 stable (non-radioactive) isotopes in nature. (The number of unstable, radioactive isotopes is in excess of 1200.) 62 of the 83 stable elements (H to Bi) have at least two isotopes; in most cases one isotope is predominant, the others being present in only trace amounts.

To a first approximation, the isotopes of an element behave almost identically, since chemical behavior is governed by the electron structure of the atom and hence on the number of protons in the nucleus.

Although this approximation is satisfactory for many experimental systems in the laboratory, it does not apply to many chemical, physical, and biological processes in nature. Differences in isotopic mass lead to subtle but significant differences in the behavior of the isotopes of an element during natural processes. This is the basis for the field of stable isotope geochemistry.

The application of stable isotope variations to geological problems has focused on the elements of low atomic weight: hydrogen, carbon, nitrogen, oxygen, and sulfur. Since the magnitude of ”isotope effects“ is proportional to the relative mass difference between isotopes (Am/m), significant isotopic variations in nature are limited to the light elements. In addition, H, C, N, 0, and S occur in relatively high abundance, participate in most important geochemical reactions, and are the most important elements in biological systems. Table 1-1 lists the average abundance of the isotopes of these elements as