Terme de Glossaire

The atomic nuclei of chemical elements have a certain number of protons and neutrons. For instance, the hydrogen nucleus contains a single proton and no neutrons. But there are also two other “varieties” of hydrogen (i.e. isotopes) that stand out from the former owing to the number of neutrons in the nucleus: deuterium, whose nucleus is composed of a proton and a neutron, and tritium, whose nucleus only has one proton but also two neutrons. The different isotopes of the same chemical element are therefore atoms whose nuclei contain the same number of protons but a different number of neutrons. Since the number of electrons is equal to the number of protons, it therefore does not vary from one isotope to another. In other words, the isotopes of the same element often have identical physical and chemical properties. However, some can be unstable: this results in nuclear decay, turning the atoms into showers of radioactive particles.

In nature, an element is often composed of a mixture of isotopes. This is the case of carbon, for instance, which is essentially carbon 12 (6 protons and 6 neutrons) but is mixed with small quantities of radioactive carbon, i.e. carbon 14 (6 protons and 8 neutrons). It is its disintegration over time (it gradually disappears) that makes it possible to determine the age of a carbon sample in which it is present.