Isotopes of lithium

Naturally occurring lithium (chemical symbol Li) (standard atomic mass: 6.941(2) atomic mass units, a.m.u.) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being by far the more abundant one: about 92.5 percent of the atoms. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon when compared with the adjacent lighter and heavier elements, helium and beryllium. This means that solely among the stable light elements, lithium can release energy through nuclear fission. The most stable radioisotope of lithium is lithium-8, which has a half-life of just 838 milliseconds. Lithium-9 has a half-life of 178 milliseconds, and lithium-11 has a half-life of about 8.6 milliseconds. All of the remaining isotopes of lithium have half-lives that are smaller than 10 nanoseconds. The shortest-lived known isotope of lithium is lithium-4 which decays by proton emission with a half-life of about 7.6x10⁻²³ seconds.

Lithium-7 and lithium-6 are two of the primordial nuclides that were produced in the Big Bang. A small percentage of lithium-6 is also known to 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 (heavy, heavy hydrogen) is produced.

Isotopes

Colex separation

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

This is the basis of the colex (column exchange) separation method, in which a counter-flow of amalgam and hydroxide passes 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 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.

The lithium should be fairly evenly distributed between the two relatively immiscible solvents. The whole system should be chemically stable, and a rapid exchange of the lithium should occur between the two phases when they are in contact.

Vacuum distillation

Lithium is heated to a temperature of about 550 celsius 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-4

Lithium-4 contains three protons and one neutron. This is the shortest-lived known isotope of lithium. It decays by proton emission to helium-3 with half-life of about 10⁻²³ seconds. 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. Lithium-6 acts as a fermion in interactions with other particles because it has three protons, three neutrons, and three electrons, and these give the atom a total atomic "spin" of plus or minus 1/2 - and not the integral spin of a boson.

Lithium-7

Lithium-7 is by far the most-common isotope of natural lithium, making up about 92.5 percent of the atoms. Any lithium-7 atom contains three protons, four neutrons, and three electrons, and it is a boson, which means that its total atomic spin is an integer, usually zero. In the Universe, because of its nuclear properties, lithium-7 is less-common than helium, beryllium, carbon, nitrogen, or oxygen, even though the latter four all have heavier nuclei than that of lithium.

The lithium that is left over from the production of (the desirable) lithium-6, which is enriched in lithium-7 and depleted in lithium-6, has been sold commercially, and some of it has been released into the environment. The relative abundance of lithium-7 as high as 35 percent greater than the natural value have 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 dicharged lithium that can range from about 6.94 atomic mass units to about 7.00 a.m.u. Hence in this example, 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.

Lithium-7 finds one use 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) 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.

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, protons, and pions.

Lithium-11

Lithium-11 is thought to possess a halo nucleus consisting of a core of 3 protons and 8 neutrons, 2 of which have a nuclear halo. It has an exceptionally large cross-section of 3.16 fm, comparable to that of ²⁰⁸Pb. It decays by β^- to ¹¹Be, which then decays in several ways (see table below).

Lithium-12

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

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)
	excitation energy
4Li	3	1	4.02719(23)	91(9)×10−24 s [6.03 MeV]	p	3He	2-
5Li	3	2	5.01254(5)	370(30)×10−24 s [~1.5 MeV]	p	4He	3/2-
6Li	3	3	6.015122795(16)	Stable			1+	[0.0759(4)]	0.07714-0.07225
7Li	3	4	7.01600455(8)	Stable			3/2-	[0.9241(4)]	0.92275-0.92786
8Li	3	5	8.02248736(10)	840.3(9) ms	β-	8Be	2+
9Li	3	6	9.0267895(21)	178.3(4) ms	β-, n (50.8%)	8Be	3/2-
					β- (49.2%)	9Be
10Li	3	7	10.035481(16)	2.0(5)×10−21 s [1.2(3) MeV]	n	9Li	(1-,2-)
10m1Li	200(40) keV			3.7(15)×10−21 s			1+
10m2Li	480(40) keV			1.35(24)×10−21 s			2+
11Li	3	8	11.043798(21)	8.75(14) ms	β-, n (84.9%)	10Be	3/2-
					β- (8.07%)	11Be
					β-, 2n (4.1%)	9Be
					β-, 3n (1.9%)	8Be
					β-, fission (1.0%)	7He, 4He
					β-, fission (.014%)	8Li, 3H
					β-, fission (.013%)	9Li, 2H
12Li	3	9	12.05378(107)#	<10 ns	n	11Li

Notes

• The precision of the abundance of isotopes of lithium and the overall atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
• Exceptional samples of lithium from geology are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass might exceed the stated value for such samples.
• Commercially available samples of lithium may have been subjected to the undisclosed or inadvertent separation of the isotopes. Substantial deviations from the given atomic mass and isotopic composition can be found.
• In depleted lithium (with the Li-6 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 atomic mass units to 7.00 a.m.u.
• The values marked with # are not purely from experimental data, but they are partly or totally estimated from the general trends. The values of spin 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 from the norm, except for the isotopic compositions and standard atomic masses from the IUPAC that use larger uncertainties.
• The unusual isotope lithium-11 has a nuclear halo of two weakly linked neutrons that explains the important difference in its nuclear radius.

References

• Isotope masses from:
  • 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. 
• 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.
  • 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. 
  • 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. 

External links

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

Isotopes of helium	Isotopes of lithium	Isotopes of beryllium
Table of nuclides