Heavy water and water. Molecular and nuclear models of heavy water (left, D2O) and water (right, H2O). Water molecules consist of an oxygen atom (red, O) bonded to two hydrogen atoms (grey, H). Oxygen's atomic nucleus consist of 6 neutrons and six protons. In heavy water, the hydrogen is replaced by deuterium, an isotope of hydrogen. Deuterium has one neutron (pink) and one proton (blue) in its nucleus, whereas hydrogen has no neutrons, just a single proton. Around 1 in 6420 atoms of hydrogen are deuterium, which is used in nuclear reactors ('heavy water'), medical tracers, and nuclear spectroscopy.