After hydrogen, the second lightest chemical element. Its atomic nucleus consists of two protons and, ordinarily, two neutrons (“helium-4”); such helium nuclei are also called alpha particles. Another variety of helium, helium-3, has only one neutron in its nucleus.

In the context of general relativity, both helium-3 and helium-4 are is of interest as two species of light atomic nuclei that formed in the early universe during Big Bang Nucleosynthesis.