Isotopes of hydrogen

The three most stable isotopes of hydrogen: protium (A = 1), deuterium (A = 2), and tritium (A = 3).

Hydrogen (H) (relative atomic mass: 1.00794) 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 (10⁻²¹ 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 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.)

Hydrogen-1 (protium)

Protium, the most common isotope of hydrogen, consists of one proton and one electron. Unique among all stable isotopes, it has no neutrons. (see diproton for a discussion of why others do not exist)

¹H (atomic mass 1.00782504(7) 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 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)

A deuterium atom contains one proton, one neutron, and one electron

²H (atomic mass 2.013553212724(78) 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 deuteron. 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 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)

A tritium atom contains one proton, two neutrons, and one electron

³H (atomic mass 3.0160492 u) 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.

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 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 into hydrogen-3 (tritium) with a half-life of about 139 yoctoseconds ((1.39 ± 0.10) × 10⁻²² 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 × 10⁻²² 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 (2.9×10⁻²² 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 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 23 yoctoseconds (2.3×10⁻²³ seconds).

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	Daughter Isotope(s)	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
1H	1	0	1.00782503207(10)	Stable			​1⁄2+	0.999885(70)	0.999816–0.999974
2H	1	1	2.0141017778(4)	Stable			1+	0.000115(70)	0.000026–0.000184
3H	1	2	3.0160492777(25)	12.32(2) y	β−	3He	​1⁄2+	Trace
4H	1	3	4.02781(11)	1.39(10)×10−22 s [4.6(9) MeV]	n	3H	2−
5H	1	4	5.03531(11)	>9.1×10−22 s ?	2n	3H	(​1⁄2+)
6H	1	5	6.04494(28)	2.90(70)×10−22 s [1.6(4) MeV]	3n	3H	2−#
					4n	2H
7H	1	6	7.05275(108)#	2.3(6)×10−23 s#	4n	3H	​1⁄2+#

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.
• 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

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.

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

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

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

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

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

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

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

See also

• Isotopes of helium
• Hydrogen isotope biogeochemistry

References

Notes

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