Common
Isotopes of helium
Helium (2He) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived; the only particle-bound ones are 6He and 8He with half-lives 806.9 and 119.5 milliseconds. In Earth's atmosphere, the ratio of 3He to 4He is 1.37×10−6. However, the isotopic abundance of helium varies greatly depending on its origin, though helium-4 is always in great preponderance. In the Local Interstellar Cloud, the proportion of 3He to 4He is 1.62(29)×10−4, which is about 120 times higher than in Earth's atmosphere. Rocks from Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. Equal mixtures of liquid 3He and 4He below 0.8 K separate into two immiscible phases due to differences in quantum statistics: 4He atoms are bosons...
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