Deuterium (Symbol: 2H, earlier, D) (from the Greek: δεύτερος (deúteros) = “the second”) is a stable isotope of hydrogen with a relative atomic mass of 1.00782519 mu and a nuclear spin of 1. Mass spectrometry and various scattering experiments reveal two elemental particles in its atomic nucleus, a positively charged proton and an uncharged neutron. These observations gave rise to its name. Other natural isotopes of hydrogen are Protium (1H) (one proton, 2.01410222 mu, nuclear spin ½) and Tritium (3H) (one proton two neutrons, 3.01610497 mu, nuclear spin ½, radioactive) (Greenwood and Earnshaw 1984). Because of its mass, deuterium is also called “heavy hydrogen.”