An isotope is a variation of a basic element. This means that one particular atom can have different “flavors”. Isotope “flavors” of the same chemical element always have the same number of protons (atomic number), but might have a different number of neutrons in its nucleus than is typical. For example, one of the most common elements is carbon. Three isotopes of carbon are carbon-12, carbon-13, and carbon-14. The number after carbon represents the mass of each isotope respectively. Each of these isotopes has 6 protons, therefore an atomic number of 6. To determine the number of neutrons just subtract the atomic number from each mass.
We call carbon-13 the heavy isotope and carbon-12 the light isotope. “Lighter” isotopes can run faster through chemical reactions and “heavier” ones lag behind. Because the different isotopes move at different speeds through plants and animals, the amounts of light and heavy isotopes can change. For instance, plants use carbon dioxide in photosynthesis. The plant will use carbon dioxide with carbon-12 faster than it will use carbon-13. Therefore, leaves in plants have less carbon-13 in the, than in the carbon dioxide. This process is called isotope fractionation.