Nuclides with atomic number of 2 but with different mass numbers
Isotopes of helium (2He)
Main isotopes[1]
Decay
abundance
half-life (t1/2)
mode
product
3He
0.0002%
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stable
4He
99.9998%
Preview warning: Infobox He isotopes: Abundance percentage not recognised "na=99.9998%" cat#%
stable
Standard atomic weight Ar°(He)
4.002602±0.000002[2]
4.0026±0.0001 (abridged)[3]
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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.
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.
^Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
^"Standard Atomic Weights: Helium". CIAAW. 1983.
^Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
^"helium-3 | chemical isotope | Britannica". www.britannica.com. Retrieved 2022-03-20.
^Busemann, H.; Bühler, F.; Grimberg, A.; Heber, V. S.; Agafonov, Y. N.; Baur, H.; Bochsler, P.; Eismont, N. A.; Wieler, R.; Zastenker, G. N. (2006-03-01). "Interstellar Helium Trapped with the COLLISA Experiment on the MiR Space Station—Improved Isotope Analysis by In Vacuo Etching". The Astrophysical Journal. 639 (1): 246. Bibcode:2006ApJ...639..246B. doi:10.1086/499223. ISSN 0004-637X. S2CID 120648440.
^Cite error: The named reference heliumfundamentals was invoked but never defined (see the help page).
^The Encyclopedia of the Chemical Elements. p. 264.
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