ABSTRACT
Approximately 1700 different isotopes are known, of which around 275 are stable. The remainder are radioactive; that is, their nuclear configurations are unstable and can change to more stable forms by nuclear transformations that are collectively known as radioactive decay. These radioactive decay processes are accompanied by the emission of particles and/or photons from the nucleus. Isotopes (or nuclides) are distinguished by the number of protons and neutrons (collectively known as nucleons) they contain and are commonly designated using mass number (A: number of protons+ neutrons) and atomic number (Z: number of protons). For example, is an isotope of carbon in which the nucleus contains 14 nucleons, of which six are protons. The proton number defines the chemical identity of the atom, since the proton charge must be balanced by the appropriate number of electrons, but it also duplicates the information provided by the chemical symbol and, in practice, is often omitted, hence 14C. Differences in the neutron number may control the stability, or otherwise, of a nucleus but have only subtle effects on chemistry, although these can be exploited in studies of stable isotope fractionation in natural systems, for example 2H/1H, 13C/12C, 15N/14N, 17O/16O, 34S/32S (see Chap. 9).
