Atmospheric hydrogen is a mixture of three isotopes. The most common is called protium (mass no. 1, atomic mass 1.007822); the protium nucleus (protium ion) is a proton. A second isotope of hydrogen is deuterium (mass no. 2, atomic mass 2.0140), the so-called heavy hydrogen, often represented in chemical formulas by the symbol D. The deuterium nucleus, or ion, is called the deuteron; it consists of a proton plus a neutron. The two isotopes are found in atmospheric hydrogen in the proportion of about 1 atom of deuterium to every 6,700 atoms of protium. Protium and deuterium differ slightly in their chemical and physical properties; for example, the boiling point of deuterium is about 3°C lower than protium. The properties of compounds they form differ depending on the ratio of the two isotopes present.
Deuterium oxide (D2O), the so-called heavy water, is present in ordinary water; the concentration of deuterium oxide is increased by electrolysis of the water. The melting point (3.79°C), boiling point (101.4°C), and specific gravity (1.107 at 25°C) of deuterium oxide are higher than those of ordinary water. Deuterium oxide is used as a moderator in nuclear reactors. Deuterium is also of importance because of the wide use it has found in scientific research; for example, chemical reaction mechanisms have been studied by the use of deuterium atoms as tracers (i.e., deuterium is substituted for atoms of ordinary hydrogen in compounds), making it possible to follow the course of individual molecules in a reaction.
Tritium (mass no. 3, atomic mass 3.016), a third hydrogen isotope, is a radioactive gas with a half-life of about 121/4 years; it is often represented in chemical formulas by the symbol T. It is produced in nuclear reactors and occurs to a very limited extent in atmospheric hydrogen. It is used in the hydrogen bomb, in luminous paints, and as a tracer. The tritium nucleus, or ion, is called the triton; it consists of a proton plus two neutrons. Tritium oxide (T2O) has a melting point (4.49°C) higher than that of deuterium oxide.
Besides being a mixture of three isotopes, hydrogen is a mixture of two forms, an ortho form and a para form, which differ in their electronic and nuclear spins. At room temperature atmospheric hydrogen is about 3/4 ortho -hydrogen and 1/4 para -hydrogen. The two forms differ slightly in their physical properties.