section_path: Properties/Isotopes
section_name: Isotopes
paragraph_text: Isotopes
  
  Hydrogen has three naturally occurring isotopes, denoted H, H and H. Other, highly unstable nuclei (H to H) have been synthesized in the laboratory but not observed in nature.
  
  Unique among the elements, distinct names are assigned to its isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and T (instead of H and H) are sometimes used for deuterium and tritium, but the symbol P was already used for phosphorus and thus was not available for protium. In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry (IUPAC) allows any of D, T, H, and H to be used, though H and H are preferred.
  
  The exotic atom muonium (symbol Mu), composed of an anti muon and an electron, can also be considered a light radioisotope of hydrogen. Because muons decay with lifetime 2.2 µs, muonium is too unstable for observable chemistry. Nevertheless, muonium compounds are important test cases for quantum simulation, due to the mass difference between the antimuon and the proton, and IUPAC nomenclature incorporates such hypothetical compounds as muonium chloride (MuCl) and sodium muonide (NaMu), analogous to hydrogen chloride and sodium hydride respectively.