ABSTRACT
Here we look at some of the principles underlying the use of isotopes in studies of the environment. These require an appreciation of what isotopes are (see 4.2), and the nature of radioactivity (see 24). The key points are as follows. The different isotopes of a given element have the same number of protons (the defining feature of an element) but different numbers of neutrons; they consequently differ in atomic mass. Most chemical elements have more than one isotope, of which one is usually overwhelmingly predominant. Most naturally occurring isotopes are stable (i.e. not radioactive), but some are unstable, that is they are radioactive, and are referred to as radio-isotopes or radionuclides. There are recognized standards for the proportion of each isotope in a sample of any given element, although in practice the relative abundance of isotopes varies depending on the source of the sample and its history. The relative amount of two isotopes in a sample is known as the isotopic ratio.
