deuterium vs. hydrogen

noun

Chemistry.
  1. an isotope of hydrogen, having twice the mass of ordinary hydrogen; heavy hydrogen. 2 H, D; 2.01; 1.

noun

  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.