Isotopes of lithium

Naturally occurring lithium (3Li) 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 839.4 milliseconds. Lithium-9 has a half-life of 178.3 milliseconds, and lithium-11 has a half-life of about 8.75 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 9.1×10−23 seconds, although the half-life of lithium-3 is yet to be determined, and is likely to be much shorter, like helium-2 (diproton) which undergoes proton decay within 10−9 s. 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. 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. 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.