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). 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. 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 7.58043×10−23 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.
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 whereas 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.
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%. A multi-stage process may be used to obtain higher degrees of separation.
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 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.
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 finds a use 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) versus 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.
Lithium-7 hydroxide is used for alkalizing of the coolant in pressurized water reactors.
* The precision of the isotope abundances and atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
* 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).