Helium

Helium is found in the Earth's atmosphere, 5.24 ppm, and in igneous rocks in the Earth's crust, 3 · 10⁻³ ppm [4]. Most of the helium on Earth is formed by radioactive decay of much heavier elements (alpha particles) and is found trapped along with natural gas [8].

The isotopes found in nature are ³He (1.38 · 10⁻⁴ %, stable) and ⁴He (99.999826 %, stable) [4].

In liquid form, helium is found in two forms. He I above ~2.2 K and He II below ~2.2 K. He II is a superconducting liquid with a very low viscosity [4].