Deuterium, which forms the heavy heart of hydrogen bombs, was discovered on this day in 1931. In the 1920s, scientists had predicted that there might be heftier versions of hydrogen floating about. Harold Urey, a chemist at Columbia University, speculated that an isotope lugging around an extra neutron would weigh enough to be distilled from normal hydrogen. Urey and two colleagues then detected deuterium by its atomic spectrum in residue distilled from liquid hydrogen. Three years later, this accomplishment earned Urey the Nobel Prize for Chemistry.
In the 1950s, deuterium was used in thermonuclear weapons because nuclear fusion of deuterium atoms (or of deuterium and the heavier hydrogen isotope, tritium) releases tremendous energy. Nuclear reactors often use deuterium oxide to control reactions, because the liquid slows down neutrons without capturing many. Chemists also use deuterium as an isotopic tracer to spy on chemical and biochemical reactions involving hydrogen.
[Source: Encyclopedia Britannica.]