Isotopes are atoms of an element that contain different numbers of neutrons in their atomic nucleus. They have the same atomic number, i.e., same number of protons (of course an equal number of electrons) but differ in their atomic mass due to the neutron numbers. For example, the three isotopes of hydrogen have one proton and one electron each but contain zero (hydrogen), one (deuterium), or two (tritium) neutrons. Chemical properties of any element are determined by its electronic configuration (and atomic number) and therefore, all the isotopes of that element react similarly. Isotopes provide an excellent tool for the study of reactions because they are isoelectronic and isosteric. This feature makes isotopes eminently suitable as tracers in enzyme research. Both radioactive and stable isotopes commonly used in enzymology are listed in Table 24.1. While remarkably similar in their chemical reactivity, their mass differences inflict subtle but definite changes in the rate of bond forming/breaking events involving them. These “isotope effects” —valuable in understanding enzyme mechanisms—are discussed in a subsequent section (Chap. 25 ).