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Heavy water (deuterium oxide (2H 2O) or D 2O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in regular water.
Standard heavy water (D2O) has the same oxygen isotopic composition but 100% deuterium and molar mass 20.027508 g mol-1 [IAPWS]. With stocks of VSMOW being used up, they have been succeeded by VSMOW2 , a standardized artificial pure salt-free water isotopic mixture made to deliver the same isotopic concentrations.