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Nuclides with atomic number of 3 but with different mass numbers

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. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon (5332.3312 for 6Li and 5606.4401 for 7Li) when compared with the adjacent lighter and heavier elements, helium (7073.9156 for helium 4) and beryllium (6462.6693 for beryllium 9). The longest lived radioisotope of lithium is 8Li, which has a half life of just 838.7. 9Li has a half life of 178.2, and 11Li has a half life of 8.75. All of the remaining isotopes of lithium have half lives that are shorter than 10 nanoseconds. The shortest lived known isotope of lithium is 4Li, which decays by proton emission with a half life of about 91 (9.1), although the half life of 3Li is yet to be determined, and is likely to be much shorter, like 2He (helium 2, diproton) which undergoes proton emission within s.

Both 7Li and 6Li are two of the primordial nuclides that were produced in the Big Bang, with 7Li to be 10−9 of all primordial nuclides, and 6Li around 10−13. A small percentage of 6Li 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 7Li. This results in some enrichment of 6Li in geological processes. In nuclear physics, 6Li is an important isotope, because when it is bombarded with neutrons, tritium is produced. Both 6Li and 7Li isotopes show nuclear magnetic resonance effect, 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. 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).

Lithium 3, also known as the triproton, would consist of three protons and zero neutrons. It was reported as proton unbound in 1969, but this result was not accepted and its existence is thus unproven. No other resonances attributable to Lithium 3 have been reported, and it is expected to decay by prompt proton emission (much like the diproton, Helium 2).

Lithium 4 contains three protons and one neutron. It is the shortest lived known isotope of lithium, with a half life of 91 (9.1) and decays by proton emission to helium 3. Lithium 4 can be formed as an intermediate in some nuclear fusion reactions.

Lithium 7 is by far 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 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. The isotopic composition of lithium in normal materials can vary somewhat depending on its origin, which determines its relative atomic mass in the source material. An accurate relative 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 and protons.

Lithium 8 has been proposed as a source of 6.4 MeV electron antineutrinos generated by the inverse beta decay to Beryllium 8. The ISODAR particle physics collaboration describes a scheme to generated Lithium 8 for immediate decay by bombarding stable Lithium 7 with 60 MeV protons created by a cyclotron particle accelerator.

Lithium 11 is a halo nucleus consisting of a lithium 9 core surrounded by two loosely bound neutrons; both neutrons must be present in order for this system to be bound, which has led to the description as a "Borromean nucleus". While the proton root mean square radius of 11Li is 2.18, its neutron radius is much larger at 3.34; for comparison, the corresponding figures for 9Li are 2.076 for the protons and 2.4 for the neutrons. It decays by beta emission and neutron emission to Beryllium 10, Beryllium 11, or Beryllium 9 (see tables above and below). Having a magic number of 8 neutrons, Lithium 11 sits on the first of five known islands of inversion, which explains its longer half life compared to adjacent nuclei.

Lithium 12 has a considerably shorter half life. It decays by neutron emission into Lithium 11, which decays as mentioned above.

While β− decay into isotopes of beryllium (often combined with single or multiple neutron emission) is predominant in heavier isotopes of lithium, Lithium 10 and Lithium 12 decay via neutron emission into Lithium 9 and Lithium 11 respectively due to their positions beyond the neutron drip line. Lithium 11 has also been observed to decay via multiple forms of fission. Isotopes lighter than Lithium 6 decay exclusively by proton emission, as they are beyond the proton drip line. The decay modes of the two isomers of Lithium 10 are unknown.