Helium

Helium (He) is the most common element in the Universe after hydrogen. In the atmosphere of the earth its concentration is only 0.0005%, while in natural gas from the interior of the earth the concentration varies from 0.4% to 8%. Natural helium exists in two isotopic forms, ⁴He (99.999%) and ³He (0.001%). ³He can be produced in appreciable amounts in nuclear reactors.