Natural abundance refers to the amount of a particular isotope of an element that is found naturally on Earth. Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. For example, carbon has two stable isotopes: carbon-12 (with 6 neutrons) and carbon-13 (with 7 neutrons). The natural abundance of these isotopes tells us how much of each isotope is present in a natural sample of carbon.