Deuterium is a non-radioactive isotope of hydrogen characterized by an extra proton. It is an odorless, flammable gas that exists in a diatomic state. The properties of deuterium and deuterium oxide are different from those of ordinary hydrogen and water. For example, the melting points of H₂ and D₂ are, respectively, -434.6 °F (- 259.2° C) and -425.9 °F (-254.4° C). The boiling points of H₂O and D₂O are, respectively, 212° F (100.0° C) and 214.52° F (101.4 ° C).

The discovery of isotopes by Frederick Soddy in 1913 spurred a special interest in the simplest chemical element, hydrogen. Isotopes of most elements differ from each other by only a few percent in their atomic weight, but an isotope of hydrogen would differ by at least 100% from ordinary hydrogen-1. The lightest possible isotope would consist of a proton and a neutron, for a mass of...