Deuterium

Deuterium is an isotope of hydrogen, the first element. Deuterium has one proton and one neutron. Hydrogen does not have a neutron, only a proton. Another isotope of hydrogen, tritium, has two neutrons. The chemical symbol for Deuterium is ²H but D is also used often.

Two deuterium atoms combined with an oxygen atom is sometimes called "heavy water." This is because it is like water (H₂O), but heavier because deuterium has one more neutron in its nucleus. Heavy water is sometimes used in nuclear reactors. It is also used as a solvent for NMR spectroscopy. This is because it will dissolve the sample, like normal water, but it will not be detected by the magnet in an ¹H NMR.