isotope

An isotope is a subclass of an element with a specific number of nucleons (protons and neutrons), i.e., a specific nuclide. The number of protons (atomic number) determines the element, and for that element, the number of neutrons, or equivalently the total of protons and neutrons (mass number) determines the element's isotope. Some isotopes (unstable isotopes or radioactive isotopes) do not last long, e.g., undergo beta decay frequently enough that their half-life is short. Stable isotopes are those that last a long time, and are ultimately more common after increasing amounts of the unstable ones have disappeared. The ratio of the amount of an unstable isotope with its stable result can be used to determine ages of material, i.e., how long since some particular ratio was established (radioactive dating).

A typical notation uses the element's symbol with a "left-side superscript" (i.e., superscript preceding the symbol) to indicate the mass number and (optionally, for clarity) a "left-side subscript" to indicate the atomic number. This notation is used, for example, in descriptions of nuclear reactions (e.g., nucleosynthesis). For example (the notation of the common stable isotope of helium, with two protons and two neutrons):

4
 He
2

Spectral lines differ by isotope, e.g., the analog to the ¹H 21-cm line is 92-cm for ²H, the hydrogen isotope that includes a neutron. That is a dramatic case: for many lines, the wavelength-difference is so small as to be indistinguishable, e.g., obscured by line broadening. However, when the difference can be distinguished, spectrography can distinguish specific isotopes.