Isotopes of lithium

Naturally occurring lithium (Li) (standard atomic mass: 6.941(2) u) is composed of two stable isotopes, ⁶
Li
and ⁷
Li
, the latter being the more abundant (92.5% natural abundance). Both natural isotopes have anomalously low nuclear binding energy per nucleon compared to the next lighter and heavier elements helium and beryllium, which means that alone among stable light elements, lithium can produce net energy through nuclear fission. Nine radioisotopes have been characterized with 3 to 13 nucleons, the most stable being ⁸
Li
with a half-life of 838 ms, ⁹
Li
with a half-life of 178.3 ms, and ¹¹Li with a half-life of 8.59 milliseconds. All of the remaining radioactive isotopes have half-lives that are shorter than 10 ns. The shortest-lived isotope of lithium is ⁴
Li
which decays through proton emission and has a half-life of 7.58x10^-23 (43) seconds.

⁷
Li
is one of the primordial nuclides, produced in Big Bang nucleosynthesis (a small amount of ⁶
Li
is also produced in stars). Lithium isotopes fractionate substantially during a wide variety of natural processes, including mineral formation (chemical precipitation), metabolism, and ion exchange. Lithium ion substitutes for magnesium and iron in octahedral sites in clay minerals, where ⁶
Li
is preferred to ⁷
Li
, resulting in enrichment of the light isotope in processes of hyperfiltration and rock alteration.

Isotopes

Colex separation

⁶Li has a greater affinity for mercury than does ⁷Li. When a lithium-mercury amalgam is in contact with a lithium hydroxide solution, ⁶Li preferentially concentrates in the amalgam, and ⁷Li in the hydroxide.

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 ⁶Li fraction is preferentially drained by the mercury whereas the ⁷Li fraction flows preferentially with the hydroxide.

At the bottom of the column, the lithium (enriched in ⁶Li) is separated from the amalgam, the mercury is recovered and reused with fresh feedstock. At the top, the lithium hydroxide solution is electrolyzed to liberate the ⁷Li-enriched 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 °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 ⁶Li atoms have a greater mean free path, they are collected preferentially.

The theoretical separation efficiency is about 8%. A multi-stage process may be used to obtain higher degrees of separation.

Lithium-4

Lithium-4 contains 3 protons and one neutron. It is the shortest-lived isotope of lithium. It decays by proton emission to ³He and has a half-life of 9.1×10⁻²³ s. It can be formed as an intermediate in some nuclear fusion reactions.

Lithium-6

Lithium-6 is valued as a source material for tritium production and as a neutron absorber in nuclear fusion. Natural lithium contains about 7.5 percent ⁶Li. Large amounts of ⁶Li have been isotopically fractionated for use in nuclear weapons. The production has by now ceased, but a stockpile remains. The isotope acts as a fermion in interactions with other particles, since it has 3 protons, 3 neutrons, and 3 electrons.

Lithium-7

Some of the material remaining from the production of ⁶Li, which is depleted in ⁶Li and enriched in ⁷Li, is made commercially available, and some has been released into the environment. Relative ⁷Li abundances as high as 35.4% greater than the natural value have been measured in ground water from a carbonate aquifer underlying West Valley Creek, Pennsylvania, down-gradient from a lithium processing plant. In depleted material, the relative ⁶
Li
abundance may be reduced to as little as 80% of its normal value, giving an atomic mass for the substance a range from about 6.94 u to about than 6.99 u. As a result, the isotopic composition of lithium is highly variable depending on its source. An accurate relative atomic mass cannot be given representatively for all samples.

⁷Li finds a use as a constituent of the solvent lithium fluoride in liquid-fluoride nuclear reactors. Indeed, the large neutron absorption cross-section of ⁶Li (941 barns, thermal) versus the small neutron absorption cross-section of ⁷Li (0.045 barns, thermal) make strict isotopic separation of lithium a requirement for fluoride reactor use.

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

Some ⁷Li has been produced which contains a lambda particle, while an atomic nucleus is generally assumed to contain only neutrons and protons.

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^- excited nucleus of ¹¹Be, which then decays in several ways (see table below).

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
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	3	3	6.015122795(16)	Stable			1+	[0.0759(4)]	0.07714-0.07225
7 Li	3	4	7.01600455(8)	Stable			3/2-	[0.9241(4)]	0.92275-0.92786
8 Li	3	5	8.02248736(10)	840.3(9) ms	β-, fission	2 4 He	2+
9 Li	3	6	9.0267895(21)	178.3(4) ms	β-, n (50.8%)	8 Be	3/2-
					β- (49.2%)	9 Be
10 Li	3	7	10.035481(16)	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	3	8	11.043798(21)	8.75(14) ms	β-, n (84.9%)	10 Be	3/2-
					β- (8.07%)	11 Be
					β-, 2n (4.1%)	9 Be
					β-, 3n (1.9%)	8 Be
					β-, fission (1.0%)	7 He , 4 He
					β-, fission (.014%)	8 Li , 3 H
					β-, fission (.013%)	9 Li , 2 H
12 Li	3	9	12.05378(107)#	<10 ns	n	11 Li

Notes

• The precision of the isotope abundances and atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
• Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
• Commercially available materials may have been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations from the given mass and composition can occur.
• In depleted material, the relative ⁶
  Li
  abundance may be reduced by as much as 80% of its normal value, giving the atomic mass a range from 6.94 u to more than 6.99 u.
• 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.
• ¹¹
  Li
  has a nuclear halo of two weakly linked neutrons, thus explaining an important difference in the 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" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • 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. {{cite journal}}: Unknown parameter |laysummary= ignored (help)
• 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" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. {{cite web}}: Check date values in: |accessdate= (help)
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide (ed.). CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0849304859. {{cite book}}: Unknown parameter |nopp= ignored (|no-pp= suggested) (help)