Isotopes of lithium

Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being far more abundant: about 92.5 percent of the atoms. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon (~5.3 MeV) when compared with the adjacent lighter and heavier elements, helium (~7.1 MeV) and beryllium (~6.5 MeV). The longest-lived radioisotope of lithium is lithium-8, which has a half-life of just 839.4 milliseconds. Lithium-9 has a half-life of 178.3 milliseconds, and lithium-11 has a half-life of about 8.75 milliseconds. All of the remaining isotopes of lithium have half-lives that are shorter than 10 nanoseconds. The shortest-lived known isotope of lithium is lithium-4, which decays by proton emission with a half-life of about 9.1×10−23 seconds, although the half-life of lithium-3 is yet to be determined, and is likely to be much shorter, like helium-2 (diproton) which undergoes proton decay within 10−9 s.

Main isotopes of lithium (3Li)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
6Li 7.59% stable
7Li 92.41% stable
6Li content may be as low as 1.9% in
commercial samples. 7Li would therefore
have a content of up to 98.1%.
Standard atomic weight Ar, standard(Li)
  • [6.938, 6.997][1]
  • Conventional: 6.941

Lithium-7 and lithium-6 are two of the primordial nuclides that were produced in the Big Bang, with lithium-7 to be 10−9 of all primordial nuclides and amount of lithium-6 around 10−13.[2] A small percentage of lithium-6 is also known to be produced by nuclear reactions in certain stars. The isotopes of lithium separate somewhat during a variety of geological processes, including mineral formation (chemical precipitation and ion exchange). Lithium ions replace magnesium or iron in certain octahedral locations in clays, and lithium-6 is sometimes preferred over lithium-7. This results in some enrichment of lithium-7 in geological processes.

Lithium-6 is an important isotope in nuclear physics because when it is bombarded with neutrons, tritium is produced.

A chart showing the abundances of the naturally-occurring isotopes of lithium.

List of isotopes
Nuclide[3]
[n 1]		 Z N Isotopic mass (Da)[4]
[n 2][n 3]		 Half-life

[resonance width]		 Decay
mode
[n 4]		 Daughter
isotope
[n 5]		 Spin and
parity
[n 6][n 7]		 Natural abundance (mole fraction)
Excitation energy Normal proportion Range of variation
3
Li
 3 0 3.030775#[5] p 2
He
4
Li
 3 1 4.02719(23) 91(9)×10−24 s
[6.03 MeV]		 p 3
He
 2−
5
Li
 3 2 5.01254(5) 370(30)×10−24 s
[~1.5 MeV]		 p 4
He
 3/2−
6
Li
[n 8]		 3 3 6.0151228874(15) Stable 1+ 0.0759(4) 0.07225–0.07714
6m
Li
 3562.88(10) keV 5.6(14)×10−17 s IT 6
Li
 0+
7
Li
[n 9]		 3 4 7.016003437(5) Stable 3/2− 0.9241(4) 0.92275–0.92786
8
Li
 3 5 8.02248625(5) 839.40(36) ms β− 8
Be
[n 10]		 2+
9
Li
 3 6 9.02679019(20) 178.3(4) ms β−, n (50.8%) 8
Be
[n 11]		 3/2−
β− (49.2%) 9
Be
10
Li
 3 7 10.035483(14) 2.0(5)×10−21 s
[1.2(3) MeV]		 n 9
Li
 (1−, 2−)
10m1
Li
 200(40) keV 3.7(15)×10−21 s 1+
10m2
Li
 480(40) keV 1.35(24)×10−21 s 2+
11
Li
[n 12]		 3 8 11.0437236(7) 8.75(14) ms β−, n (86.3%) 10
Be
 3/2−
β− (5.978%) 11
Be
β−, 2n (4.1%) 9
Be
β−, 3n (1.9%) 8
Be
[n 13]
β−, α (1.7%) 7
He
, 4
He
β−, fission (.009%) 8
Li
, 3
H
β−, fission (.013%) 9
Li
, 2
H
12
Li
 3 9 12.05261(3) <10 ns n 11
Li
13
Li
 3 10 13.06117(8) 3.3(12)×10−21 s 2n 11
Li
 3/2−#
  1. mLi – Excited nuclear isomer.
  2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. Modes of decay:
    IT:Isomeric transition
    n:Neutron emission
    p:Proton emission
  5. Bold symbol as daughter – Daughter product is stable.
  6. ( ) spin value – Indicates spin with weak assignment arguments.
  7. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  8. One of the few stable odd-odd nuclei
  9. Produced in Big Bang nucleosynthesis and by cosmic ray spallation
  10. Immediately decays into two α-particles for a net reaction of 8Li → 24He + e−
  11. Immediately decays into two α-particles for a net reaction of 9Li → 24He + 1n + e−
  12. Has 2 halo neutrons
  13. Immediately decays into two 4He atoms for a net reaction of 11Li → 24He + 31n + e−
  • In depleted lithium (with the 6Li removed), the relative abundance of lithium-6 can be reduced to as little as 20 percent of its normal value, giving the measured atomic mass ranging from 6.94 Da to 7.00 Da.

Isotope separation

Colex separation
Main article: COLEX process (isotopic separation)

Lithium-6 has a greater affinity than lithium-7 for the element mercury. When an amalgam of lithium and mercury is added to solutions containing lithium hydroxide, the lithium-6 becomes more concentrated in the amalgam and the lithium-7 more in the hydroxide solution.

The colex (column exchange) separation method makes use of this by passing a counter-flow of amalgam and hydroxide through a cascade of stages. The fraction of lithium-6 is preferentially drained by the mercury, but the lithium-7 flows mostly with the hydroxide. At the bottom of the column, the lithium (enriched with lithium-6) is separated from the amalgam, and the mercury is recovered to be reused with fresh raw material. At the top, the lithium hydroxide solution is electrolyzed to liberate the lithium-7 fraction. The enrichment obtained with this method varies with the column length and the flow speed.

Vacuum distillation

Lithium is heated to a temperature of about 550 °C in a vacuum. Lithium atoms evaporate from the liquid surface and are collected on a cold surface positioned a few centimetres above the liquid surface. Since lithium-6 atoms have a greater mean free path, they are collected preferentially.

The theoretical separation efficiency is about 8.0 percent. A multistage process may be used to obtain higher degrees of separation.

Lithium-3

Lithium-3, also known as the triproton, would consist of three protons and zero neutrons. It was reported as proton unbound in 1969, but this result was not accepted and its existence is thus unproven.[6] No other resonances attributable to 3Li have been reported, and it is expected to decay by prompt proton emission (much like the diproton, 2He).[7]

Lithium-4

Lithium-4 contains three protons and one neutron. It is the shortest-lived known isotope of lithium, with a half-life of about 91 yoctoseconds, 9.1×10−23 seconds and decays by proton emission to helium-3.[5] Lithium-4 can be formed as an intermediate in some nuclear fusion reactions.

Lithium-6

Lithium-6 is valuable as the source material for the production of tritium (hydrogen-3) and as an absorber of neutrons in nuclear fusion reactions. Natural lithium contains about 7.5 percent lithium-6, with the rest being lithium-7. Large amounts of lithium-6 have been separated out for placing into hydrogen bombs. The separation of lithium-6 has by now ceased in the large thermonuclear powers, but stockpiles of it remain in these countries.

The D-T fusion reaction (between deuterium and tritium) has been investigated as a possible energy source, as it is currently the only fusion reaction with sufficient energy output for feasible implementation. In this scenario, enriched lithium-6 would be required to generate the necessary quantities of tritium. The abundance of lithium-6 is a potential limiting factor in this scenario, though other sources of lithium (such as seawater) may also be usable.[8]

Lithium-6 is one of only three stable isotopes with a spin of 1, the others being deuterium and nitrogen-14,[9] and has the smallest nonzero nuclear electric quadrupole moment of any stable nucleus.

Lithium-7

Lithium-7 is by far the most abundant isotope, making up about 92.5 percent of all natural lithium. A lithium-7 atom contains three protons, four neutrons, and three electrons. Because of its nuclear properties, lithium-7 is less common than helium, beryllium, carbon, nitrogen, or oxygen in the Universe, even though the latter four all have heavier nuclei.

The industrial production of lithium-6 results in a waste product which is enriched in lithium-7 and depleted in lithium-6. This material has been sold commercially, and some of it has been released into the environment. A relative abundance of lithium-7, as high as 35 percent greater than the natural value, has been measured in the ground water in a carbonate aquifer underneath the West Valley Creek in Pennsylvania, which is downstream from a lithium processing plant. In the depleted lithium, the relative abundance of lithium-6 can be reduced to as little as 20 percent of its nominal value, giving an atomic mass for the discharged lithium that can range from about 6.94 Da to about 7.00 Da. Hence, the isotopic composition of lithium can vary somewhat depending on its source. An accurate atomic mass for samples of lithium cannot be measured for all sources of lithium.[10]

Lithium-7 is used as a part of the molten lithium fluoride in molten salt reactors: liquid-fluoride nuclear reactors. The large neutron absorption cross section of lithium-6 (about 940 barns[11]) as compared with the very small neutron cross section of lithium-7 (about 45 millibarns) makes high separation of lithium-7 from natural lithium a strong requirement for the possible use in lithium fluoride reactors.

Lithium-7 hydroxide is used for alkalizing of the coolant in pressurized water reactors.[12]

Some lithium-7 has been produced, for a few picoseconds, which contains a lambda particle in its nucleus, whereas an atomic nucleus is generally thought to contain only neutrons and protons.[13][14]

Lithium-11

Lithium-11 is thought to possess a halo nucleus consisting of a core of three protons and eight neutrons, two of which are in a nuclear halo. It has an exceptionally large cross-section of 3.16 fm2, comparable to that of 208Pb. It decays by beta emission to 11Be, which then decays in several ways (see table below).

Lithium-12

Lithium-12 has a considerably shorter half-life of around 10 nanoseconds. It decays by neutron emission into 11Li, which decays as mentioned above.

Decay chains

While β− decay into isotopes of beryllium (often combined with single- or multiple-neutron emission) is predominant in heavier isotopes of lithium, 10Li and 12Li decay via neutron emission into 9Li and 11Li respectively due to their positions beyond the neutron drip line. Lithium-11 has also been observed to decay via multiple forms of fission. Isotopes lighter than 6Li decay exclusively by proton emission, as they are beyond the proton drip line. The decay modes of the two isomers of 10Li are unknown.

See also

References
  1. Meija, Juris; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. Fields, Brian D. (2011). "The Primordial Lithium Problem". Annual Review of Nuclear and Particle Science. 61 (1): 47–68. arXiv:1203.3551. Bibcode:2011ARNPS..61...47F. doi:10.1146/annurev-nucl-102010-130445. S2CID 119265528.
  3. Half-life, decay mode, nuclear spin, and isotopic composition is sourced in:
    Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  4. Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
  5. "Isotopes of Lithium". Retrieved 20 October 2013.
  6. Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001–21. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  7. Purcell, J. E.; Kelley, J. H.; Kwan, E.; Sheu, C. G.; Weller, H. R. (2010). "Energy Levels of Light Nuclei (A = 3)" (PDF). Nuclear Physics A. 848 (1): 1. Bibcode:2010NuPhA.848....1P. doi:10.1016/j.nuclphysa.2010.08.012.
  8. Bradshaw, A.M.; Hamacher, T.; Fischer, U. (2010). "Is nuclear fusion a sustainable energy form?" (PDF). Fusion Engineering and Design. 86 (9): 2770–2773. doi:10.1016/j.fusengdes.2010.11.040. hdl:11858/00-001M-0000-0026-E9D2-6.
  9. Chandrakumar, N. (2012). Spin-1 NMR. Springer Science & Business Media. p. 5. ISBN 9783642610899.
  10. Coplen, Tyler B.; Hopple, J. A.; Böhlke, John Karl; Peiser, H. Steffen; Rieder, S. E.; Krouse, H. R.; Rosman, Kevin J. R.; Ding, T.; Vocke, R. D., Jr.; Révész, K. M.; Lamberty, A.; Taylor, Philip D. P.; De Bièvre, Paul; "Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrial materials and reagents", U.S. Geological Survey Water-Resources Investigations Report 01-4222 (2002). As quoted in T. B. Coplen; et al. (2002). "Isotope-Abundance Variations of Selected Elements (IUPAC technical report)" (PDF). Pure and Applied Chemistry. 74 (10): 1987–2017. doi:10.1351/pac200274101987. S2CID 97223816.
  11. Holden, Norman E. (January–February 2010). "The Impact of Depleted 6Li on the Standard Atomic Weight of Lithium". Chemistry International. International Union of Pure and Applied Chemistry. Retrieved 6 May 2014.
  12. Managing Critical Isotopes: Stewardship of Lithium-7 Is Needed to Ensure a Stable Supply, GAO-13-716 // U.S. Government Accountability Office, 19 September 2013; pdf
  13. Emsley, John (2001). Nature's Building Blocks: An A-Z Guide to the Elements. Oxford University Press. pp. 234–239. ISBN 978-0-19-850340-8.
  14. Brumfiel, Geoff (1 March 2001). "The Incredible Shrinking Nucleus". Physical Review Focus. 7. doi:10.1103/PhysRevFocus.7.11.

Lewis, G. N.; MacDonald, R. T. (1936). "The Separation of Lithium Isotopes". Journal of the American Chemical Society. 58 (12): 2519–2524. doi:10.1021/ja01303a045.

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