lithium-6
Definition: Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 and lithium-7, 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(3) MeV for lithium-6 and 5606.4401(6) MeV for lithium-7) when compared with the adjacent lighter and heavier elements, helium (7073.9156(4) MeV for helium-4) and beryllium (6462.6693(85) MeV for beryllium-9). The longest-lived radioisotope of lithium is lithium-8, which has a half-life of just 838.7(3) milliseconds. Lithium-9 has a half-life of 178.2(4) ms, and lithium-11 has a half-life of 8.75(6) ms. 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 91(9) yoctoseconds (9.1(9)×10−23 s), 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 emission within 10−9 s.
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