Helium-6
Although there are nine known isotopes of helium (He) (relative atomic mass: 4.002602(2)), only helium-3 (3
He
) and helium-4 (4
He
) are stable. All radioisotopes are short-lived, the longest-lived being 6
He
with a half-life of 806.7 milliseconds. The least stable is 5
He
, with a half-life of 7.6×10−22 s, although it is possible that 2
He
has an even shorter half-life.
In the Earth's atmosphere, there is one 3
He
atom for every million 4
He
atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3
He
is around a hundred times higher. Rocks from the 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.
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Wikipedia