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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.
Physically and chemically the substance is similar to ordinary water, but while the hydrogen atoms in normal H2O consist of one proton and one electron, many of the hydrogen atoms in heavy-water have the added weight of a neutronâ an isotope known as deuterium.