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
Vapor pressure pressures of liquids at ambient temperatures increase with decreasing boiling points. This is illustrated in the vapor pressure chart (see right) that shows graphs of the vapor pressures versus temperatures for a variety of liquids. At the normal boiling point of a liquid, the vapor pressure is equal to the standard atmospheric pressure defined as 1 atmosphere (760 Torr or 101.325 kPa or 14.69595 PSI). For example, at any given temperature, methyl chloride has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point (−24.2 °C), which is where the