Isotopes of helium a thousand times lower than helium-4 (2.17 K). This difference is explained by quantum statistics, since helium-3 atoms are fermions, while helium-4 atoms are bosons, which condense to a superfluid more easily. The most common isotope, , is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized nuclei. is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis. Terrestrial helium consists almost exclusively (99.99986%) of this isotope. Its boiling point of 4.2 K is the lowest of
Critical points of the elements (data page) Critical points of the elements (data page) David R. Lide (ed), "CRC Handbook of Chemistry and Physics, 85th Edition", online version. CRC Press. Boca Raton, Florida, 2003; Section 6, Fluid Properties; Critical Constants. Also agrees with Celsius values from Section 4: Properties of the Elements and Inorganic Compounds, Melting, Boiling, Triple, and Critical Point Temperatures of the Elements J.A. Dean (ed), "Lange's Handbook of Chemistry" (15th Edition), McGraw-Hill, 1999; Section 6; Table 6.5 Critical Properties National Physical Laboratory, "Kaye and Laby Tables of Physical and Chemical Constants"; D. Ambrose, M.B. Ewing, M.L. McGlashan, Critical constants and second virial coefficients of