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Naturally occurring lithium (chemical symbol Li) (relative atomic mass: 6.941(2)) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being far more abundant: about 92.5 percent of the atoms. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon (~5.3 MeV) when compared with the adjacent lighter and heavier elements, helium (~7.1 MeV) and beryllium (~6.5 MeV). The longest-lived 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 shorter than 10 nanoseconds. The shortest-lived known isotope of lithium is lithium-4, which decays by proton emission with a half-life of about 6977909999999999999♠9.1×10−23 seconds, although the half-life of lithium-3 is yet to be determined, and is likely to be much shorter.
Contents
- Colex separation
- Vacuum distillation
- Lithium 4
- Lithium 6
- Lithium 7
- Lithium 11
- Lithium 12
- Notes
- Decay chains
- References
Lithium-7 and lithium-6 are two of the primordial nuclides that were produced in the Big Bang, with lithium-7 to be 10−9 of all primordial nuclides and amount of lithium-6 around 10−13. A small percentage of lithium-6 is also known to be 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 is produced.
Colex separation
Lithium-6 has a greater affinity than lithium-7 for the element mercury. When an amalgam of lithium and mercury is added to solutions containing lithium hydroxide, the lithium-6 becomes more concentrated in the amalgam and the lithium-7 more in the hydroxide solution.
The colex (column exchange) separation method makes use of this by passing a counter-flow of amalgam and hydroxide 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 be 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.
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 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. It is the shortest-lived known isotope of lithium, with a half-life of about 6977909999999999999♠9.1×10−23 seconds and decays by proton emission to helium-3. 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 is one of only three stable isotopes with a spin of 1 and has the smallest nonzero nuclear electric quadrupole moment of any stable nucleus.
Lithium-7
Lithium-7 is by far the most-common isotope, making up about 92.5 percent of all natural lithium. A lithium-7 atom contains three protons, four neutrons, and three electrons. Because of its nuclear properties lithium-7 is less common than helium, beryllium, carbon, nitrogen, or oxygen in the Universe, even though the latter four all have heavier nuclei.
The industrial production of lithium-6 results in a waste product which is enriched in lithium-7 and depleted in lithium-6. This material has been sold commercially, and some of it has been released into the environment. A relative abundance of lithium-7, as high as 35 percent greater than the natural value, has 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 discharged lithium that can range from about 6.94 Da to about 7.00 Da. Hence 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 is used 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 three protons and eight neutrons, two of which are in a nuclear halo. It has an exceptionally large cross-section of 3.16 fm2, comparable to that of 208Pb. It decays by beta emission to 11Be, which then decays in several ways (see table below).
Lithium-12
Lithium-12 has a considerably shorter half-life of around 10 nanoseconds. It decays by neutron emission into 11Li, which decays as mentioned above.
Notes
Decay chains
While β− decay into isotopes of beryllium (often combined with single- or multiple-neutron emission) is predominant over heavier isotopes of lithium, 10Li and 12Li decay via neutron emission into 9Li and 11Li respectively due to their positions above the neutron drip line. Lithium-11 has also been observed to decay via multiple forms of fission. Lighter isotopes of lithium (<6Li) are only known to decay by proton emission. The decay modes of the two isomers of 10Li are unknown.