Isotopes are nuclides (atomic species) with the same atomic number but different mass number. The elements with atomic numbers above 83 have only radioactive isotopes. The name comes from the fact that isotopes of one and the same element are in the same place in the periodic table. However, in the nuclide map they appear separately. Isotopes thus contain an equal number of protons in their atomic nuclei, but different numbers of neutrons.

As a rule, every naturally occurring element has one or a few stable isotopes, while its other isotopes are radioactive (that is, unstable) and decay sooner or later. However, there are also elements in which all isotopes are unstable.

Definition of Isotopes: Nuclides whose nuclei contain the same number of protons but different numbers of neutrons and therefore have different masses.