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, which is the smallest unit of matter that retains all the chemical properties of an element. Isotopes have unique properties that make them useful in various applications, such as nuclear medicine, oil and gas exploration, basic research, and national security. There are two main types of isotopes: stable and unstable (radioactive) . All elements have isotopes, and there are 254 known stable isotopes. Isotopes can be identified by their name or symbol, which includes the mass number (e.g., carbon-14 or C-14) or by standard notation, where A is the mass number, Z is the atomic number, and E is the element symbol. Isotope analysis is the determination of isotopic signature, the relative abundances of isotopes of a given element in a particular sample, and is frequently done by isotope ratio mass spectrometry. Another common application is isotopic labeling, the use of unusual isotopes as tracers or markers in chemical reactions.