Isotopes

Isotopes can be thought of as different ‘flavours’ of a particular element (such as oxygen or carbon), that are distinguished by the number of neutrons in their nucleus (and hence their atomic mass). Carbon for instance most commonly has a mass of 12 (written as ¹²C), but there are also a small fraction of carbon atoms with mass 13 and 14 (¹³C and ¹⁴C), similarly oxygen is normally ¹⁶O, but with small amounts of ¹⁷O and ¹⁸O. All of the isotopes of an element behave in similar way chemically. However, because the mass of each isotope is slightly different there are certain physical processes that will discriminate (or ‘fractionate’) between them. For instance, during evaporation of water, it is slightly easier for the lighter isotopes to escape from the liquid, and so water vapour generally has less ¹⁸O than the liquid water from which it came. Because of these physical effects, looking at the ratio of one isotope to another can often be very useful in tracing where these atoms came from.