Heavy water, also called deuterium oxide (D20), is composed of oxygen and two hydrogen atoms in the form of deuterium, which has about twice the mass of normal hydrogen. As a result, heavy water has a molecular weight of about 20, while ordinary water has a molecular weight of about 18. Approximately one part heavy water can be found in 6,500 parts of ordinary water, and it may be extracted by fractional distillation. It is used in thermonuclear weapons and nuclear reactors and as an isotopic tracer in studies of chemical and biochemical processes. Heavy water was discovered by Harold C. Urey (1893–1981) in 1931. He received the Nobel Prize in Chemistry in 1934 for his discovery of heavy hydrogen.