Isotopes are different types of atoms (nuclides) of the same chemical element, each having a different number of neutrons. In a corresponding manner, isotopes differ in mass number (or number of nucleons) but not in atomic number.[1] 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. The atomic 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 nuclide would 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, 146C, 13153I, and 23992U).
Some isotopes are radioactive and are therefore described as radioisotopes or radionuclides, while others have never been observed to undergo radioactive decay and are described as stable isotopes. For example, 146C is a radioactive form of carbon while 126C and 136C are stable isotopes. There are about 339 naturally occurring nuclides on Earth[2], of which 288 are primordial nuclides and 259 are "stable"[2]. However, some apparently "stable" isotopes are predicted by theory to be radioactive with very long half-lives.[citation needed] Adding in the radioactive nuclides that have been created artificially, there are more than 3100 currently known nuclides.[3]
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