Lithium-7 is the most abundant isotope of lithium, making up between 92.2% and 98.1% of all terrestrial 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, carbon, nitrogen, or oxygen in the Universe, even though the latter three all have heavier nuclei. The Castle Bravo thermonuclear test greatly exceeded its expected yield due to incorrect assumptions about the nuclear properties of lithium-7. The industrial production of lithium-6 results in a waste product which is enriched in lithium-7 and depleted in lithium-6. This lithium has been sold commercially, and some of it has been released into the environment. A relative abundance of lithium-7 as much 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. 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 and protons. == Lithium-8 ==

Isotopes of lithium