Isotopes of hydrogen

Hydrogen (₁H) has three naturally occurring isotopes: ¹H, ²H, and ³H. ¹H and ²H are stable, while ³H has a half-life of 12.32(2) years.^{[3][nb 1]} Heavier isotopes also exist; all are synthetic and have a half-life of less than 1 zeptosecond (10⁻²¹ s).^[4][5] 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: ²H is deuterium^[6] and ³H is tritium.^[7] The symbols D and T are sometimes used for deuterium and tritium; IUPAC (International Union of Pure and Applied Chemistry) accepts said symbols, but recommends the standard isotopic symbols ²H and ³H, to avoid confusion in alphabetic sorting of chemical formulas.^{[8] 1}H, with no neutrons, may be called protium to disambiguate.^[9] (During the early study of radioactivity, some other heavy radioisotopes were given names, but such names are rarely used today.)

Note: "y" means year, but "ys" means yoctosecond (10⁻²⁴ second).

¹H (atomic mass 1.007825031898(14) Da) is the most common hydrogen isotope, with an abundance of >99.98%. Its nucleus consists of only a single proton, so it has the formal name protium.

The proton has never been observed to decay, so ¹H is considered a stable isotope. Some Grand Unified Theories proposed in the 1970s predict that proton decay can occur with a half-life between 10²⁸ and 10³⁶ years.^[14] If so, then ¹H (and all nuclei now believed to be stable) are only observationally stable. As of 2018, experiments have shown that the mean lifetime of the proton is >3.6×10²⁹ years.^[15]

Deuterium, ²H (atomic mass 2.014101777844(15) Da), the other stable hydrogen isotope, has one proton and one neutron in its nucleus, called a deuteron. ²H comprises 26–184 ppm (by population, not mass) of hydrogen on Earth; the lower number tends to be found in hydrogen gas and the higher enrichment (150 ppm) is typical of seawater. Deuterium on Earth has been enriched with respect to its initial concentration in the Big Bang and outer solar system (≈27 ppm, by atom fraction) and older parts of the Milky Way (≈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 is not a significant toxicity hazard. Water enriched in ²H 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.

Tritium, ³H (atomic mass 3.016049281320(81) Da), contains one proton and two neutrons in its nucleus (triton). It is radioactive, β⁻ decaying into helium-3 with half-life 12.32(2) years.^{[nb 1][3]} Traces of ³H occur naturally due to cosmic rays interacting with atmospheric gases. ³H has also been released in nuclear tests. It is used in fusion bombs, as a tracer in isotope geochemistry, and in self-powered lighting devices.

The most common way to produce ³H is to bombard a natural isotope of lithium, ⁶Li, with neutrons in a nuclear reactor.

Tritium can be used in chemical and biological labeling experiments as a radioactive tracer.^[16][17] Deuterium–tritium fusion uses ²H and ³H as its main reactants, giving energy through the loss of mass when the two nuclei collide and fuse at high temperatures.

⁴H (atomic mass 4.02643(11)), with one proton and three neutrons, is a highly unstable isotope. It has been synthesized in the laboratory by bombarding tritium with fast-moving deuterons;^[18] the triton captured a neutron from the deuteron. The presence of ⁴H was deduced by detecting the emitted protons. It decays by neutron emission into ³H with a half-life of 139(10) ys (or 1.39(10)×10⁻²² s).

In the 1955 satirical novel The Mouse That Roared, the name quadium was given to the ⁴H that powered the Q-bomb that the Duchy of Grand Fenwick captured from the United States.

⁵H (atomic mass 5.03531(10)), with one proton and four neutrons, is highly unstable. It has been synthesized in the lab by bombarding tritium with fast-moving tritons;^[18][19] one triton 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 ⁵H deduced. It decays by double neutron emission into ³H and has a half-life of 86(6) ys (8.6(6)×10⁻²³ s) – the shortest half-life of any known nuclide.^[3]

⁶H (atomic mass 6.04496(27)) has one proton and five neutrons. It has a half-life of 294(67) ys (2.94(67)×10⁻²² s).

⁷H (atomic mass 7.05275(108)) has one 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; all six of the helium-8's neutrons were donated to the hydrogen nucleus. The two remaining protons were detected by the "RIKEN telescope", a device made of several layers of sensors, positioned behind the target of the RI Beam cyclotron.^{[5] 7}H has a half-life of 652(558) ys (6.52(558)×10⁻²² s).^[3]

⁴H and ⁵H decay directly to ³H, which then decays to stable ³He. Decay of the heaviest isotopes, ⁶H and ⁷H, has not been experimentally observed.^[11]

${\displaystyle {\begin{array}{rcl}\\{\ce {^{3}_{1}H}}&{\ce {->[12.32\ {\ce {y}}]}}&{\ce {{^{3}_{2}He}+e^{-}}}\\{\ce {^{4}_{1}H}}&{\ce {->[139\ {\ce {ys}}]}}&{\ce {{^{3}_{1}H}+{^{1}_{0}n}}}\\{\ce {^{5}_{1}H}}&{\ce {->[86\ {\ce {ys}}]}}&{\ce {{^{3}_{1}H}+{2_{0}^{1}n}}}\\{}\end{array}}}$

Decay times are in yoctoseconds (10⁻²⁴ s) for all these isotopes except ³H, which is in years.

• Hydrogen atom
• Hydrogen isotope biogeochemistry
• Hydrogen-4.1 (Muonic helium)
• Muonium – acts like an exotic light isotope of hydrogen
• Media related to Isotopes of hydrogen at Wikimedia Commons