The enrichment of one isotope relative to another in a chemical or physical process. Two isotopes of an element are different in weight but not in gross chemical properties, which are determined by the number of electrons. It can be predicted theoretically and demonstrated experimentally, however, that subtle chemical effects do result from the difference in mass of isotopes. Isotopes of an element may have slightly different equilibrium constants for a particular chemical reaction, so that fractionation of the isotopes results from that reaction. One of the assumptions of radiocarbon dating is that Carbon 12, Carbon 13, and Carbon 14 are passed around the carbon cycle at similar rates. The three isotopes are chemically very similar, but slight differences between them may cause them to be taken up at different rates by some plants and animals. This isotopic fractionation may cause inaccuracies in radiocarbon dating. Both Carbon 12 and Carbon 13 are stable isotopes and their ratio should therefore remain constant throughout life and after death. If it has changed from the expected value, then fractionation has occurred. Once the degree of fractionation is known, it can be corrected for mathematically by the laboratory.