Isotope

Chemical element with the same atomic number (same number of protons) but different atomic masses (different number of neutrons). Isotopes can be stable or radioactive. The ratio between the amounts of the stable isotope and the radioactive isotope in water can be used to date the latter (example of C14 for the best known). Mass differences between isotopes can, for their part, explain the origin of a water sample: e.g. the content of rainwater in O18 with respect to O15 decreases with altitude.