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Hydrogen with different numbers of neutrons

Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted Hydrogen 1, Hydrogen 2, and Hydrogen 3. Hydrogen 1 and Hydrogen 2 are stable, while Hydrogen 3 has a half life of 12.32 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, Hydrogen 5 is the least stable, while Hydrogen 7 is the most. Hydrogen is the only element whose isotopes have different names that remain in common use today: the Hydrogen 2 (or hydrogen 2) isotope is deuterium and the Hydrogen 3 (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 (Hydrogen 2 and Hydrogen 3) instead to avoid confusion in the alphabetic sorting of chemical formulas. The isotope Hydrogen 1, with no neutrons, may be called protium to disambiguate. (During the early study of radioactivity, some other heavy radioactive isotopes were given names, but such names are rarely used today.)

Hydrogen 2 (atomic mass 2.014101777844), 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 0.0023%, 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 Hydrogen 1 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.

Hydrogen 3 (atomic mass 3.016049281320) is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium 3 through β− decay with a half life of 12.32. D T nuclear fusion uses tritium as its main reactant, along with deuterium, liberating energy through the loss of mass when the two nuclei collide and fuse at high temperatures.

Hydrogen 4 (atomic mass 4.02643) contains one proton and three neutrons in its nucleus. It is a highly unstable isotope of hydrogen. It has been synthesized in the laboratory by bombarding tritium with fast moving deuterium nuclei. In this experiment, the tritium nucleus captured a neutron from the fast moving deuterium nucleus. The presence of the hydrogen 4 was deduced by detecting the emitted protons. It decays through neutron emission into hydrogen 3 (tritium) with a half life of 139 (or 1.39e 22). In the 1955 satirical novel The Mouse That Roared, the name quadium was given to the hydrogen 4 isotope that powered the Q bomb that the Duchy of Grand Fenwick captured from the United States.

Hydrogen 5 (atomic mass 5.03531) is a highly unstable isotope of hydrogen. The nucleus consists of a proton and four neutrons. It has been synthesized in the laboratory by bombarding tritium with fast moving tritium nuclei. In this experiment, one tritium nucleus captures two neutrons from the other, becoming a nucleus with one proton and four neutrons. The remaining proton may be detected, and the existence of hydrogen 5 deduced. It decays through double neutron emission into hydrogen 3 (tritium) and has a half life of 86 (8.6e 23) – the shortest half life of any known nuclide.

Hydrogen 6 (atomic mass 6.04496) consists of a proton and five neutrons. It has a half life of 294 (2.94e 22).

Hydrogen 7 (atomic mass 7.05275) consists of a proton and six neutrons. It was first synthesized in 2003 by a group of Russian, Japanese and French scientists at RIKEN's Radioactive Isotope Beam Factory by bombarding hydrogen with helium 8 atoms. In the resulting reaction, all six of the helium 8 neutrons were donated to the hydrogen's nucleus. The two remaining protons were detected by the "RIKEN telescope", a device composed of several layers of sensors, positioned behind the target of the RI Beam cyclotron. Hydrogen 7 has a half life of 652 (6.52e 22).

The majority of heavy hydrogen isotopes decay directly to Hydrogen 3, which then decays to the stable isotope Helium 3. Decay of the heaviest isotopes, Hydrogen 6 and Hydrogen 7, are not experimentally observed. Decay times are in yoctoseconds (e= 24) for all these isotopes except Hydrogen 3, which is expressed in years.