Views: 4 Author: Site Editor Publish Time: 2023-02-10 Origin: Site
Naturally occurring lithium consists of two stable isotopes,6Li and 7Li, the latter being more abundant (95.15% natural abundance).Both natural isotopes have unusually low nuclear binding energies per nucleon (compared to their periodic table neighbors helium and beryllium);lithium is the only one that can generate net energy through nuclear fission low number elements. The binding energy per nucleon of these two lithium nuclei is lower than that of any other stable nuclides other than hydrogen 1 , deuterium and helium 3.Thus, despite its very low atomic weight, lithium is less prevalent in the solar system than 25 of the top 32 chemical elements.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 ms.
All remaining radioisotopes have half-lives shorter than 8.6 milliseconds.The shortest-lived isotope of lithium is 4Li, which decays by proton emission with a half-life of 7.6 × 10−23 s.The 6Li isotope is one of only five stable nuclides with both an odd number of protons and an odd number of neutrons, the other four stable odd and odd nuclides are hydrogen 2, boron 10, nitrogen 14 and tantalum 180m.7Li is a primordial element (or more precisely, a primordial nuclide) produced in Big Bang nucleosynthesis.During stellar nucleosynthesis, a small amount of 6Li and 7Li is produced in the star, but it "burns up" further, as fast as it is produced. 7Li can also be produced in carbon stars.Solar wind, cosmic rays striking heavier atoms, and radioactive decay of 7Be and 10Be in the early solar system may have produced additional small amounts of 6Li and 7Li.Lithium isotopes segregate in large quantities in a variety of natural processes,including mineral formation (chemical precipitation),metabolism, and ion exchange.
Lithium ion substitution of magnesium and iron in clay mineral octahedral sites, with 6Li superior to 7Li, leads to enrichment of light isotopes during ultrafiltration and rock alteration.The exotic 11Li is known to have a neutron halo of 2 neutrons orbiting its nucleus of 3 protons and 6 neutrons.A process called laser isotope separation can be used to separate lithium isotopes, specifically 7Li from 6Li.Nuclear weapons manufacturing and other nuclear physics applications are a major source of artificial lithium fractionation, and the light isotope 6Li is retained by industrial and military reserves to such an extent that it causes slight but measurable changes in the ratio of 6Li to 7Li in natural resources such as rivers.This leads to unusually uncertain normalized atomic weights for lithium, as this number depends on the natural abundance ratios of these naturally occurring stable lithium isotopes as they are available in commercial lithium mineral resources.Both stable isotopes of lithium can be laser cooled and used to create the first quantum degenerate Bose-Fermi mixture.
Main articles:Nucleosynthesis,Stellar nucleosynthesis, and Lithium burning.Although it was synthesized in the Big Bang,lithium (along with beryllium and boron) is significantly less abundant in the universe than other elements.This is due to the relatively low temperatures of stars required to destroy lithium and the lack of a general process for producing lithium.According to modern cosmological theory, lithium in two stable isotopes (lithium-6 and lithium-7)was one of three elements synthesized in the Big Bang. Although the amount of lithium produced by big bang nucleosynthesis depends on the number of photons per baryon, lithium abundance is calculable for acceptable values, and there is a "cosmological lithium difference" in the universe: older stars Seems to contain less lithium than they should, with some younger stars having more.The lack of lithium in older stars is apparently due to lithium "mixing" into the stellar interior,where it is destroyed,whereas lithium is produced in younger stars.Although it collides with a proton to transform into two helium atoms at temperatures above 2.4 million degrees Celsius (a temperature easily reached in the interiors of most stars), lithium is more abundant in descendant stars than calculations predictLithium is also present in brown dwarf interstellar objects and certain anomalous orange stars.Because lithium is present in cooler, less massive brown dwarfs but is destroyed in hotter red dwarfs,its presence in the stellar spectrum can be used in a "lithium test" to distinguish the two, since both are smaller than the sun.Certain orange stars may also contain high concentrations of lithium.Those orange stars where lithium concentrations are found to be higher than usual (such as Centaurus X-4) orbit massive objects (neutron stars or black holes), the gravity of which would apparently pull the heavier lithium to the surface of hydrogen-helium stars, thus resulting in more lithium being observed.