In essence, isotopes are different varieties of the same kind of element. Their atoms have the same number of protons but variable numbers of neutrons, meaning that they differ from each other in terms of their atomic weight.

For example, carbon-12, carbon-13 and carbon-14 are all isotopes of carbon. The numeric modifier reflects the differing weights of each isotope. Every atom of these isotopes contains 6 protons and then 6 (¹²C), 7 (¹³C), or 8 (¹⁴C) neutrons respectively.

Isotopes can be stable, meaning that they do not decay over time, or radiogenic, meaning that they decay into isotopes of other elements over time.

Within bioarchaeology, stable isotopes of carbon and nitrogen can be used to reconstruct prehistoric diets, and stable isotopes of strontium and oxygen can be used to examine mobility in the past. Radiogenic isotopes like ¹⁴C can also be used to date organic archaeological samples that are up to 45,000 years old.

In future bioarchaeology vocab posts I’ll go over the key isotopes that are particularly important in bioarchaeology, including stable isotopes of carbon (¹²C + ¹³C), nitrogen (¹⁴N + ¹⁵N), strontium (⁸⁶Sr + ⁸⁷Sr), and the radioisotope ¹⁴C.

Image Credits: Carbon rottweiler found here. Carbon-12 rottweiler found here. Carbon-13 rottweiler found here. Carbon-14 rottweiler found here. First stable isotope cat found here, second found here. First radioactive isotope cat found here, second found here.
Chemistry cat helium joke found here.

References: If you want to learn more about isotopes, I highly recommend the following chapter, which is available for free as a pdf online (link):

Tykot, R. (2006). Isotope Analyses and the Histories of Maize Histories of maize: multidisciplinary approaches to the prehistory, linguistics, biogeography, domestication, and evolution of maize, 131-142 DOI: 10.1016/B978-012369364-8/50262-X