An isotope is a form of a chemical element whose atomic nucleus contains a specific number of neutron s, in addition to the number of proton s that uniquely defines the element. The nuclei of most atom s contain neutrons as well as protons. (An exception is the common form of hydrogen, whose nucleus consists of a lone proton).

Every chemical element has more than one isotope. For any element, one of the isotopes is more abundant in nature than any of the others, although often multiple isotopes of a single element are mixed.

Sometimes the isotope of an element is denoted by writing the nucleon number after the chemical symbol, and not as a superscript. Thus, some texts will denote carbon-14 as C-14 or C14 instead of 14C.

An isotope and/or nuclide is specified by the name of the particular element (this indicates the atomic number) followed by a hyphen and the mass number (e.g. helium-3, helium-4, carbon-12, carbon-14, uranium-235 and uranium-239).

When a chemical symbol is used, e.g. "C" for carbon, standard notation (now known as "AZE notation" because A is the mass number, Z the atomic number, and E for element) is to indicate the mass number (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 Because the atomic number is given by the element symbol, it is common to state only the mass number in the superscript and leave out the atomic number subscript.

The letter m is sometimes appended after the mass number to indicate a nuclear isomer, a metastable or energetically-excited nuclear state (as opposed to the lowest-energy ground state), for example 180m 73Ta (tantalum-180m).