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Title: Hydrogen-1  
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Subject: Helium-3, Proton, Star, Speed of sound, ATP synthase, Neutron moderator, Protonium, Isotopologue, Isotopes of helium, Swain equation
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Hydrogen (H) (Standard atomic mass: 1.00794 u) has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in the laboratory but not observed in nature. The most stable radioisotope is tritium, with a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than a zeptosecond (10-21 second). Of these, 5H is the most stable, and the least stable isotope is 7H.[1][2]

Hydrogen is the only element that has different names for its isotopes in common use today. The 2H (or hydrogen-2) isotope is usually called deuterium, while the 3H (or hydrogen-3) isotope is usually called tritium. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium. The IUPAC states that while this use is common it is not preferred. The ordinary isotope of hydrogen, 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-1 (protium)

1H (atomic mass 1.00782504(7) u) the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.

The proton has never been observed to decay and hydrogen-1 is therefore considered a stable isotope. Some recent theories of particle physics predict that proton decay can occur with a half-life of the order of 1036 years. If this prediction is found to be true, then hydrogen-1 (and indeed all nuclei now believed to be stable) are only observationally stable. To date however, experiments have shown that if proton decay occurs, the half-life must be greater than 6.6 × 1033 years.

Hydrogen-2 (deuterium)

2H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleus. Deuterium comprises 0.0026 – 0.0184% (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 23 ppm). Presumably the differential concentration of D 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 normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-NMR 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 (tritium)

3H 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 years.[3] Small amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases. Tritium has also been released during nuclear weapons tests. It is used in thermonuclear fusion weapons, as a tracer in isotope geochemistry, and specialized in self-powered lighting devices.

The most common method of producing tritium is by bombarding a natural isotope of lithium, lithium-6, with neutrons in a nuclear reactor.

Tritium was once used routinely in chemical and biological labeling experiments as a radiolabel, which has become less common in recent times. 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.


4H contains one proton and three neutrons in its nucleus. It is a highly unstable isotope of hydrogen. It has been synthesised in the laboratory by bombarding tritium with fast-moving deuterium nuclei.[4] 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. Its atomic mass is 4.02781 ± 0.00011.[5] It decays through neutron emission with a half-life of (1.39 ± 0.10) × 10−22 seconds.[6]

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.


5H is a highly unstable isotope of hydrogen. The nucleus consists of a proton and four neutrons. It has been synthesised in the laboratory by bombarding tritium with fast-moving tritium nuclei.[4][7] 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 and has a half-life of at least 9.1 × 10−22 seconds.[6]


6H decays through triple neutron emission and has a half-life of 2.90×10−22 seconds.[6] It consists of 1 proton and 5 neutrons.


7H consists of a proton and six neutrons. It was first synthesised in 2003 by a group of Russian, Japanese and French scientists at RIKEN's RI Beam Science Laboratory by bombarding hydrogen with helium-8 atoms. In the resulting reaction, all six of the helium-8's 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.[2]


Z(p) N(n) isotopic mass (u) half-life decay
Daughter Isotope(s)[n 1] nuclear
(mole fraction)[n 2]
range of natural
(mole fraction)
1H 1 0 1.00782503207(10) Stable[n 3][n 4] 12+ 0.999885(70) 0.9998160.999974
2H[n 5] 1 1 2.0141017778(4) Stable 1+ 0.000115(70)[n 6] 0.0000260.000184
3H[n 7] 1 2 3.0160492777(25) 12.32(2) y β- 3He 12+ Trace[n 8]
4H 1 3 4.02781(11) 1.39(10)×10Template:Val/delimitnum/gaps11 s
[4.6(9) MeV]
n 3H 2
5H 1 4 5.03531(11) >9.1×10Template:Val/delimitnum/gaps11 s ? n 4H (12+)
6H 1 5 6.04494(28) 2.90(70)×10Template:Val/delimitnum/gaps11 s
[1.6(4) MeV]
3n 3H 2#
4n 2H
7H 1 6 7.05275(108)# 2.3(6)×10Template:Val/delimitnum/gaps11 s# 4n 3H 12+#


  • Commercially available materials may have been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations from the given mass and composition can occur.
  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.
  • Nuclide masses are given by IUPAP Commission on Symbols, Units, Nomenclature, Atomic Masses and Fundamental Constants (SUNAMCO)
  • Isotope abundances are given by IUPAC Commission on Isotopic Abundances and Atomic Weights

See also

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General references
  • Isotope masses from:
  • Isotopic compositions and standard atomic masses from:
  • Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Further reading

Isotopes of neutronium Isotopes of hydrogen Isotopes of helium
Table of nuclides
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