Isotopes are atoms of the same element that have the same number of protons (atomic number) but a different number of neutrons, meaning that their mass number varies. They are a type of atom, the smallest unit of matter that retains all the chemical properties of an element. Isotopes can either form spontaneously (naturally) through radioactive decay of a nucleus or artificially by bombarding a stable nucleus with charged particles via accelerators or neutrons in a nuclear reactor.

Isotopes have unique properties that make them useful in various applications such as nuclear medicine, oil and gas exploration, basic research, and national security. They are important in diagnostics and treatment applications. Isotope analysis is the determination of isotopic signature, the relative abundances of isotopes of a given element in a particular sample. Isotopic labeling is another common application, which is the use of unusual isotopes as tracers or markers in chemical reactions.