Naturally occurring lithium (Li) (standard atomic mass: 6.941(2) u) is composed of two stable isotopes (⁶Li and ⁷Li, the latter being the more abundant (92.5% natural abundance). Seven radioisotopes have been characterized, the most stable being ⁸Li with a half-life of 838 ms and ⁹Li with a half-life of 178.3 ms. All of the remaining radioactive isotopes have half-lives that are shorter than 8.6 ms. The shortest-lived isotope of lithium is ⁴Li which decays through proton emission and has a half-life of 7.580429×10^-23 s.

⁷Li is one of the primordial elements or, more properly, primordial isotopes, produced in Big Bang nucleosynthesis (a small amount of ⁶Li 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 ⁶Li is preferred to ⁷Li, resulting in enrichment of the light isotope in processes of hyperfiltration and rock alteration.

Isotopes

Colex Separation

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

At the bottom of the column, the Lithium (enriched in Lithium-6) 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 Lithium-7 enriched fraction. The enrichment obtained with this method varies with the column length and the flow speed.

This method leads to mercury pollution lost in wastes, spills, and through evaporation.

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 cm above the liquid surface. Lithium-6 atoms have a greater mean free path, they are collected preferentially.

The theoretical separation efficiency is about 8%. 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 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 lithium-6. Large amounts of lithium-6 have been isotopically fractionated for use in nuclear weapons.

Lithium-7

Some of the material remaining from the production of lithium-6, which is depleted in lithium-6 and enriched in lithium-7, is made commercially available, and some has been released into the environment. Relative lithium-7 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 (USA), down-gradient from a lithium processing plant. In depleted material, the relative ⁶Li 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. 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.

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

Table

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 ⁶Li 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.
• ¹¹Li has a Nuclear halo of two weakly linked neutrons, thus explaining an important difference in the radius.

References

• Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
• Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
• Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
  • Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
  • National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
  • David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.

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