Isotopes (Greek isos = "equal", tópos = "site, place") are any of the different types of atoms (nuclides) of the same chemical element, each having a different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons, the number of protons plus neutrons. A nuclide is any particular atomic nucleus with a specific atomic number Z and mass number A; it is equivalently an atomic nucleus with a specific number of protons and neutrons. Collectively, all the isotopes of all the elements form the set of nuclides. The distinction between the terms isotope and nuclide has somewhat blurred, and they are often used interchangeably. Isotope is better used when referring to several different nuclides of the same element; nuclide is more generic and is used when referencing only one nucleus or several nuclei of different elements. For example, it is more correct to say that an element such as fluorine consists of one stable nuclide rather than that it has one stable isotope. Isotopes and nuclides are specified by the name of the particular element, implicitly giving the atomic number, followed by a hyphen and the mass number (e.g. helium-3, carbon-12, carbon-13, iodine-131 and uranium-238). In symbolic form, the number of nucleons is denoted as a superscripted prefix to the chemical symbol (e.g. 3He, 12C, 13C, 131I and 238U). About 339 nuclides occur naturally on Earth, of which 256 (about 75%) are stable (or, to be careful, have never been observed to decay; this note is necessary because many "stable" isotopes are predicted to be radioactive with very long half-lives). Counting the radioactive nuclides not found in nature that have been created artificially, more than 3100 nuclides are currently known.
A neutral atom has the same number of electrons as protons. Thus, different...
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