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Deuterium. Heavy water is water (H ₂ O) in which oxygen is bound to atoms of the hydrogen isotope deuterium ( ² H). Heavy water is so named because it is significantly more dense (>1.1 g/cm ³ ) than ordinary ("light") water, 1 H ₂ O (1 gm/cm ³ ). Heavy water is not radioactive and has the same chemical properties as light water; a person could drink a glass of heavy water without harm. However, heavy water is better than light water at moderating (slowing) neutrons, which makes it useful in some nuclear reactor cores. ^{To Mods: wrong thread, IK, but in the next update i'd like to suggest Tritium as an element[properties below.]}  

Deuterium and tritium. All hydrogen atoms have atomic number 1, that is, one proton in the nucleus; common or light hydrogen also has mass number 1, that is, its nucleus consists solely of a lone proton. Deuterium ( ² H) has atomic number 1 and mass number 2, because its nucleus contains one proton plus one neutron. The presence of the neutrons in the deuterium atoms of heavy water is what makes it "heavy" (i.e., more dense than common water). Tritium ( ³ H) is an isotope of hydrogen whose nuclei contain one proton plus two neutrons. Tritium can also combine with oxygen to form heavy water, but tritium is much rarer than deuterium, so virtually all heavy water consists of ² H ₂ O (deuterium oxide). Tritium heavy water is radioactive and has been used as a tracer in certain biological experiments.