- Isotope of hydrogen with a neutron as well as a proton in its nucleus, discovered by Harold Urey (1893–1981) and of importance to both nuclear technology and astrophysics
- An isotope of hydrogen whose nucleus contains a neutron and a proton. It is a highly explosive and flammable gas. It is not radioactive, and is used, for instance, to bombard atomic nuclei. Its symbol is D. Also called heavy hydrogen (1).
- symbol D
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