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Deuterium (D or 2H) is a hydrogen isotope with a nucleus of a proton and neutron rather than the more common isotope with just a proton as the nucleus. A water molecule including two deuterium atoms is called heavy water (D2O), and with one deuterium atom, semi-heavy water (HDO). Heavy water is about 10% heavier than typical water. The effect of the additional neutron on chemistry is very slight: generally just slight changes in the speed of the reaction or the equilibrium between two compounds, but the differences can be used in observation to discover abundances, e.g., to study hydrology. For example, deuterium's equivalent to the hydrogen 21-cm line has a wavelength of 96 cm, a useful difference for astronomy. Deuterium is often abbreviated D, which is sometimes used as a variable designating a quantity related to deuterium, such as the mass fraction of something. For example, Dp (p for "primal") has been used to indicate the mass fraction of deuterium produced by the Big Bang nucleosynthesis.
Tritium (3H) is the hydrogen isotope with two neutrons. It is radioactive, decaying into helium-3 with a half-life of 12.32 years, thus very rare in nature.