Isotopes of hydrogen
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.
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 formulae. 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.)
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.
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.
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. 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.
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. 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. It decays through neutron emission with a half-life of (1.39 ± 0.10) × 10−22 seconds.
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 and has a half-life of at least 9.1 × 10−22 seconds.
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. Hydrogen-7 has a half life of 2.3×10−23 seconds.
|Z(p)||N(n)||isotopic mass (u)||half-life||decay
|Daughter Isotope(s)[n 1]||nuclear
(mole fraction)[n 2]
|range of natural
|1H||1||0||1.00782503207(10)||Stable[n 3][n 4]||1⁄2+||0.999885(70)||0.999816–0.999974|
|2H[n 5]||1||1||2.0141017778(4)||Stable||1+||0.000115(70)[n 6]||2.6×10−5–0.000184|
|3H[n 7]||1||2||3.0160492777(25)||12.32(2) y||β-||3He||1⁄2+||Trace[n 8]|
|5H||1||4||5.03531(11)||>9.1×10−22 s ?||2n||3H||(1⁄2+)|
- Bold for stable isotopes
- Refers to that in water.
- Greater than 6.6×1033 yr. See proton decay.
- This and 3He are the only stable nuclides with more protons than neutrons
- Produced during Big Bang nucleosynthesis
- Tank hydrogen has a 2H abundance as low as 3.2×10−5 (mole fraction).
- Produced during Big Bang nucleosynthesis, but not primordial, as all such atoms have since decayed to 3He
- 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
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.
- Y. B. Gurov et al. (2004). "Spectroscopy of superheavy hydrogen isotopes in stopped-pion absorption by nuclei". Physics of Atomic Nuclei 68 (3): 491–497. Bibcode:2005PAN....68..491G. doi:10.1134/1.1891200.
- A. A. Korsheninnikov et al. (2003). "Experimental Evidence for the Existence of 7H and for a Specific Structure of 8He". Physical Review Letters 90 (8): 082501. Bibcode:2003PhRvL..90h2501K. doi:10.1103/PhysRevLett.90.082501.
- G. L. Miessler, D. A. Tarr (2004). Inorganic Chemistry (3rd ed.). Pearson Prentice Hall. ISBN 978-0-13-035471-6.
- G. M. Ter-Akopian et al. (2002). "Hydrogen-4 and Hydrogen-5 from t+t and t+d transfer reactions studied with a 57.5-MeV triton beam". AIP Conference Proceedings 610: 920. doi:10.1063/1.1470062.
- "The 2003 Atomic Mass Evaluation". Atomic Mass Data Center. Retrieved 2008-11-15.
- G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties". Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
- A. A. Korsheninnikov et al. (2001). "Superheavy Hydrogen 5H". Physical Review Letters 87 (9): 92501. Bibcode:2001PhRvL..87i2501K. doi:10.1103/PhysRevLett.87.092501.
- General references
- Isotope masses from:
- 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 and P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683.
- M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. 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.
- National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005.
- 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.
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