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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.
1 Deuterium (the hydrogen isotope with one neutron) can form water with oxygen. This is called heavy water as deuterium has twice the mass of normal hydrogen (protium). 2  There are 254 known stable isotopes and 80 elements which have at least one stable isotope.  Twenty-six elements only have one stable isotope.