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an isotope of hydrogen, having twice the mass of ordinary hydrogen; heavy hydrogen. 2 H, D; 2.01; 1.
a colorless, odorless, flammable gas that combines chemically with oxygen to form water: the lightest of the known elements. H; 1.00797; 1; density: 0.0899 grams/liter at 0°C and 760 millimeters pressure.
