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Hydrogen (H) has three naturally occurring isotopes, designated ¹H, ²H and ³H. These isotopes are also known by their common names. The isotope ²H is referred to as deuterium or heavy hydrogen, while ³H is known as tritium or super-heavy hydrogen. The simplest isotope, ¹H, which has no neutrons in its nucleus, is called protium. Deuterium, sometimes represented by the chemical symbol D, is neither radioactive nor toxic. Water molecules containing deuterium atoms instead of protium are known as heavy water (D₂O). Heavy water is used as a neutron moderator and coolant in nuclear reactors.
