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.
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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.
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 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 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.
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.[1]
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).
| nuclide symbol |
Z(p) | N(n) | isotopic mass (u) |
half-life | decay mode(s)[2] |
daughter isotope(s)[n 1] |
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[n 2] |
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[n 3] |
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[n 4] |
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[n 5] |
||||||||
| 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 |
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| Book: Lithium | |
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| Isotopes of helium | Isotopes of lithium | Isotopes of beryllium |
| Index to isotope pages · Table of nuclides | ||