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 given that, the number of neutrons, or equivalently the total of protons and neutrons (mass number) determines the isotope of that element. 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 the unstable ones disappear. 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.
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