Isotopes of hydrogen

Hydrogen (H) (standard atomic weight: [1.00784, 1.00811], conventional 1.008) has three naturally occurring isotopes, sometimes denoted ¹H, ²H, and ³H. The first two of these are stable while ³H has a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than one zeptosecond (10^âˆ'21 second). Of these, ⁵H is the most stable, and ⁷H is the least.

Hydrogen is the only element whose isotopes have different names that are 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 in the 2005 Red Book that while the use of D and T is common, it is not preferred because it can cause problems in the alphabetic sorting of chemical formulas. 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 1.007825032241(94) u) is 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 formal name protium.

The proton has never been observed to decay, and hydrogen-1 is therefore considered a stable isotope. Some grand unified theories proposed in the 1970s predict that proton decay can occur with a half-life between 10³¹ and 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 the minimum proton half-life is in excess of 10³⁴ years.

Hydrogen-2 (deuterium)

²H (atomic mass 2.01410177811(12) u), 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 â€" 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 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-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 (atomic mass 3.01604928199(23) u) is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through βâˆ' decay with a half-life of 12.32 years. Trace 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.

Hydrogen-4

⁴H (atomic mass is 4.02643(11) u) 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. 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 about 139 ± 10 yoctoseconds, or (1.39 ± 0.10) × 10^âˆ'22 seconds.

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

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. 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 at least 910 yoctoseconds (9.1 × 10^âˆ'22 seconds).

Hydrogen-6

⁶H decays either through triple neutron emission into hydrogen-3 (tritium) or quadruple neutron emission into hydrogen-2 (deuterium) and has a half-life of 290 yoctoseconds (2.9 × 10^âˆ'22 seconds).

Hydrogen-7

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 Radioactive Isotope Beam Factory 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. Hydrogen-7 has a half life of 23 yoctoseconds (2.3×10^âˆ'23 seconds).

List of isotopes

Notes

• 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.
• Isotope abundances are given by IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW)

Decay chains

The majority of heavy hydrogen isotopes decay directly to ³H, which then decays to the stable isotope ³He. However, ⁶H has occasionally been observed to decay directly to stable ²H.

${\displaystyle \mathrm {{}_{1}^{3}H} \ {\xrightarrow {\ \mathrm {12.32\ y} }}\ \mathrm {{}_{2}^{3}He} +\mathrm {e{}_{}^{-}} }$

${\displaystyle \mathrm {{}_{1}^{4}H} \ {\xrightarrow {\ \mathrm {139\ ys} }}\ \mathrm {{}_{1}^{3}H} +\mathrm {{}_{0}^{1}n} }$

${\displaystyle \mathrm {{}_{1}^{5}H} \ {\xrightarrow {\ \mathrm {>910\ ys} }}\ \mathrm {{}_{1}^{3}H} +\mathrm {2{}_{0}^{1}n} }$

${\displaystyle \mathrm {{}_{1}^{6}H} \ {\xrightarrow {\ \mathrm {290\ ys} }}\ \mathrm {{}_{1}^{3}H} +\mathrm {3{}_{0}^{1}n} }$

${\displaystyle \mathrm {{}_{1}^{6}H} \ {\xrightarrow {\ \mathrm {290\ ys} }}\ \mathrm {{}_{1}^{2}H} +\mathrm {4{}_{0}^{1}n} }$

${\displaystyle \mathrm {{}_{1}^{7}H} \ {\xrightarrow {\ \mathrm {23\ ys} }}\ \mathrm {{}_{1}^{3}H} +\mathrm {4{}_{0}^{1}n} }$

Note that the decay times are in yoctoseconds for all isotopes except ³H, which is expressed in years.

See also

• Hydrogen isotope biogeochemistry
• Muonium

References

Notes

General references

• Isotope masses from:
  • W.J.Huang; G.Audi; M.Wang; F.G.Kondev; S.Naimi; X.Xu (2017). "The Ame2016 atomic mass evaluation (I). Evaluation of input data; and adjustment procedures" (PDF). Chinese Physics C. 41 (3): 7. Bibcode:2017ChPhC..41c0002H. doi:10.1088/1674-1137/41/3/030002. 
  • M.Wang; G.Audi; F.G.Kondev; W.J.Huang; S.Naimi; X.Xu (2017). "The Ame2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 7. Bibcode:2017ChPhC..41c0003W. doi:10.1088/1674-1137/41/3/030003. 
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683â€"800. doi:10.1351/pac200375060683. 
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051â€"2066. doi:10.1351/pac200678112051. Lay summary. 
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3â€"128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. Archived from the original (PDF) on 2008-09-23. 
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved 23 February 2017. 
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9. 

Further reading

• B. Dumé (7 March 2003). "Hydrogen-7 makes its debut". Physics World.