Isotopes of helium
Nuclides with atomic number of 2 but with different mass numbers / From Wikipedia, the free encyclopedia
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Although there are nine known isotopes of helium (2He) (standard atomic weight: 4.002602(2)), only helium-3 (3
He
) and helium-4 (4
He
) are stable.[4] All radioisotopes are short-lived, the longest-lived being 6
He
with a half-life of 806.92(24) milliseconds. The least stable is 10
He
, with a half-life of 260(40) yoctoseconds (2.6(4)×10−22 s), although it is possible that 2
He
may have an even shorter half-life.
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Standard atomic weight Ar°(He) | |||||||||||||||||||||
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In the Earth's atmosphere, the ratio of 3
He
to 4
He
is 1.343(13)×10−6.[5] However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of 3
He
to 4
He
is 1.62(29)×10−4,[6] which is 121(22) times higher than that of atmospheric helium. 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.[7] The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.
Equal mixtures of liquid 3
He
and 4
He
below 0.8 K separate into two immiscible phases due to differences in quantum statistics: 4
He
atoms are bosons while 3
He
atoms are fermions.[8] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.