Glossary term: Isotope

Description: Atomic nuclei consist of neutrons and protons. The number of protons in a nucleus is the so-called "atomic number", which determines the corresponding chemical element: Atoms with a single proton are hydrogen atoms, those with six protons are carbon, and so on. Atomic nuclei that have the same number of protons, but different numbers of neutrons, are called isotopes of the chemical element in question. Ordinary hydrogen nuclei only have a single proton, and no neutron at all. A nucleus with one proton and one neutron is still hydrogen, but "heavy hydrogen", also called deuterium. Typically, only a few isotopes of a given element are stable. The others will decay radioactively into more stable nuclei.

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