A helion is a short name for the naked nucleus of helium, a doubly positively charged helium ion. In practice, helion refers specifically the nucleus of the helium-3 isotope, consisting of two protons and one neutron. The nucleus of the other stable isotope of helium, helium-4 isotope, which consists of two protons and two neutrons, is specifically called an alpha particle.
This particle is emitted in the decay of tritium, a isotope of hydrogen:
According to CODATA, the mass of a helion particle is 5.006 412 700(62) × 10−27 kg.