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Isotopes are different types of atoms (nuclides) of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in atomic mass and in mass number. The difference in the number of nucleons comes from a difference how many neutronsare in the atomic nucleus. The number of protons (the atomic number) is the same because that is what characterizes a chemical element. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14, respectively. Theatomic number of carbon is 6, so the neutron numbers in these isotopes of carbon are therefore 12−6 = 6, 13−6 = 7, and 14–6 = 8, respectively.
A nuclide is an atomic nucleus with a specified composition of protons and neutrons. The nuclide concept emphasizes nuclear properties over chemical properties while the isotope concept does the converse; for the neutron number has drastic effects on nuclear properties but negligible effects on chemical properties. Since isotope is the older term, it is better known, and it is still sometimes used in contexts where nuclidewould be more proper, such as nuclear technology.
An isotope or nuclide is specified by the name of the particular element (this indicates the atomic number implicitly) followed by a hyphen and the mass number (e.g. helium-3, carbon-12, carbon-13, iodine-131 and uranium-238). When a chemical symbol is used, e.g., "C" for carbon, standard notation is to indicate the number of nucleons with a superscript at the upper left of the chemical symbol and to indicate the atomic number with a subscript at the lower left (e.g. 32He, 126C, 136C, 13153I, and 23892U).
There are about 339 naturally occurring nuclides on Earth, of which 288 are primordial nuclides and 269 are "stable". To be precise, the nuclides termed "stable" are nuclides which have never been observed to decay. This qualification is necessary because many "stable" isotopes are predicted to be radioactive with very long half-lives. Adding in the radioactive nuclides that have been created artificially, there are more than 3100 currently known nuclides.