Heavy water contains what isotopic form of hydrogen?

Heavy water contains deuterium, the heavier stable isotope of hydrogen.

Ordinary hydrogen is mostly protium, whose nucleus contains one proton and no neutrons. Deuterium has one proton and one neutron, giving it roughly twice the atomic mass of protium. When hydrogen atoms in water are replaced by deuterium, the result is deuterium oxide, written D₂O.

Chemist Harold Urey discovered deuterium in 1931, earning the 1934 Nobel Prize in Chemistry. Heavy water occurs naturally in tiny amounts, but it can be concentrated for industrial use. It has played an important role as a neutron moderator in some nuclear reactors because deuterium slows neutrons without absorbing as many as ordinary hydrogen does.

A common mix-up is tritium, another hydrogen isotope. Tritium has one proton and two neutrons and is radioactive, while deuterium is stable. Heavy water is also not radioactive simply because it contains deuterium.