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(subclass of an element with a specific number of nucleons)

An isotope is a subclass of an element with a specific number of nucleons, i.e., protons and neutrons. 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.

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).

Example: notation of the common stable isotope of helium (two protons and two neutrons):



Referenced by:
Advanced Composition Explorer (ACE)
alpha capture
Big Bang nucleosynthesis (BBN)
beta decay
Bose-Einstein statistics
calcium (Ca)
carbon (C)
methane (CH4)
carbon monoxide (CO)
dalton (Da)
deuterium (D)
fractionation factor
helium (He)
iron (Fe)
potassium/thorium ratio (K/Th ratio)
lithium (Li)
lanthanide (Ln)
Moon formation
nickel (Ni)
nitrogen (N)
oxygen (O)
proton-proton chain
radioactive dating
radioactive decay
radioactive heating
silicon (Si)
superluminous supernova (SLSN)
solar neutrino unit (SNU)
sodium (Na)
Solar and Heliospheric Observatory (SOHO)
sulfur (S)
valley of beta stability