Hydrogen-1 followed by deuterium
Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. 1H and 2H are stable, while 3H has a half-life of 12.32(2) years. Heavier isotopes also exist, all of which are synthetic and have a half-life of less than one zeptosecond (10−21 s).
Of these, 5H is the least stable, while 7H is the most.
Hydrogen is the only element whose isotopes have different names that remain in common use today: the 2H (or hydrogen-2) isotope is deuterium and the 3H (or hydrogen-3) isotope is tritium. The symbols D and T are sometimes used for deuterium and tritium. The IUPAC accepts the D and T symbols, but recommends using standard isotopic symbols (2H and 3H) instead to avoid confusion in the alphabetic sorting of chemical formulas. The isotope 1H, with no neutrons, is sometimes called protium. (During the early study of radioactivity, some other heavy radioactive isotopes were given names, but such names are rarely used today.)Hydrogen-2 (deuterium)
2H (atomic mass 2.014101777844(15) Da), the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleus. The nucleus of deuterium is called a deuteron. Deuterium comprises 0.0026–0.0184% (26 ppm or 184 ppm ; by population, not by mass) of hydrogen samples on Earth, with the lower number tending to be found in samples of hydrogen gas and the higher enrichment (0.015% or 150 ppm) typical of ocean water. Deuterium on Earth has been enriched with respect to its initial concentration in the Big Bang and the outer solar system (about 27 ppm, by atom fraction) and its concentration in older parts of the Milky Way galaxy (about 0.023%, or 23 ppm). Presumably the differential concentration of deuterium in the inner solar system is due to the lower volatility of deuterium gas and compounds, enriching deuterium fractions in comets and planets exposed to significant heat from the Sun over billions of years of solar system evolution.
Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of protium is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-nuclear magnetic resonance spectroscopy. Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.