Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 (6Li) and lithium-7 (7Li), with the latter being far more abundant on Earth. Radioisotopes are short-lived: the particle-bound ones, 8Li, 9Li, and 11Li, have half-lives of 838.7, 178.2, and 8.75 milliseconds respectively. Both of the natural isotopes have anomalously low nuclear binding energy per nucleon (5332.3312(3) keV for 6Li and 5606.4401(6) keV for 7Li) when compared with the adjacent lighter and heavier elements, helium (7073.9156(4) keV for helium-4) and beryllium (6462.6693(85) keV for beryllium-9), and so their synthesis requires non-equilibrium conditions. Both 7Li and 6Li were produced in the Big Bang, with 7Li estimated to be 5×10−10 of all primordial matter, and 6Li around 10−14 (undetectable). This difference is significant because both isotopes of lithium are efficiently destroyed by protons, while beryllium-7 is not and subsequently decays to lithium. A portion of 7Li is also known to be formed in certain stars (red giants), called the Cameron–Fowler mechanism; while beryllium-7 is a normal product of nuclear burning, it can only contribute to lithium production if it is convected to the surface before it decays. Thus, it is considered that almost all 6Li, like much 7Li, is cosmogenic and produced by spallation. The isotopes of lithium separate somewhat during a variety of geological processes, including mineral formation (chemical precipitation and ion exchange) – for example, lithium ions replace magnesium or iron in certain octahedral locations in clays, and 6Li is sometimes preferred over 7Li, resulting in enrichment of the clays. It is considered that an accurate relative atomic mass for samples of lithium cannot be measured for all sources of lithium. In nuclear physics, 6Li is an important isotope, because when it is exposed to slow neutrons, tritium is produced with nearly 100% yield; contrarily, 7Li is almost unreactive with slow neutrons. Both 6Li and 7Li isotopes show nuclear magnetic resonance, despite being quadrupolar (with nuclear spins of 1+ and 3/2−). 6Li has sharper lines, but due to its lower abundance requires a more sensitive NMR-spectrometer. 7Li is more abundant, but has broader lines because of its larger nuclear spin and quadrupole. The range of chemical shifts is the same of both nuclei and lies within +10 (for LiNH2 in liquid NH3) and −12 (for Li+ in fulleride).
List of isotopes
Isotope separation
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, and the lithium-7 retained 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.
Other methods In the vacuum distillation technique, 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 higher velocity at the same temperature (due to lower mass), they evaporate preferentially and, if no gaseous collisions occur, are collected in the same ratio (i.e. the mean free path should be large compared to the distance). The theoretical separation efficiency of this method is about 8.0 percent, the square root of the mass ratio. A multistage process may be used to obtain higher degrees of separation. The isotopes of lithium, in principle, can also be separated through electrochemical methods or distillation chromatography, which are currently under research.
Lithium-5
Lithium-5 is very short-lived (< 10−21 s), decaying into a proton and helium-4. It is formed as an intermediate in the fusion of deuterium and helium-3:
2 D +
3 H e ⟶
5 L i ∗ ⟶
4 H e + p + 18.4 M e V {\displaystyle {}^{2}\mathrm {D} +{}^{3}\mathrm {He} \longrightarrow {}^{5}\mathrm {Li} ^{*}\longrightarrow {}^{4}\mathrm {He} +\mathrm {p} +18.4\ \mathrm {MeV} }
The reaction is greatly enhanced by the existence of a resonance. Lithium-5, which has a natural spin state of −3/2 at the 0 MeV ground state, has a +3/2 excited spin state at 16.66 MeV. Because the reaction creates lithium-5 nuclei with an energy level close to this state, it happens more frequently. A symmetrical resonance in the helium-5 nucleus makes the deuterium–tritium fusion reaction the most favourable known.
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