Hydrogen gas consists of diatomic (two-atom) molecules, with the formula H2. It is the lightest of all known substances. There is only about 0.05 part per million of hydrogen gas in the air. It rises to the top of the atmosphere and is lost into space. It is continually being replaced by volcanic gases, by the decay of organic matter, and from coal deposits, which still contain some of the hydrogen from when they were decaying organic matter.
There are three isotopes of hydrogen, two stable and one radioactive. Like all isotopes, they have the same number of protons in the nucleus (in this case, one) but differing numbers of neutrons. Hydrogen is the only element whose isotopes go by their own names: protium (used only occasionally, when it is necessary to distinguish it from the others), deuterium, and tritium. Their mass numbers are one, two, and three, respectively. Protium, the most common hydrogen isotope, constitutes 99.985% of all hydrogen atoms; it has no neutrons in its nuclei. Deuterium, the other stable isotope, has one neutron in its nucleus; it constitutes 0.015% of all hydrogen atoms—that's about one out of every 6,700 atoms. Water made out of deuterium instead of protium is called heavy water; it is used as a moderator—a slower of neutrons—in nuclear reactors. Tritium has two neutrons and is radioactive, with a half-life of 12.33 years. In spite of its short lifetime, it remains present in the atmosphere in very tiny amounts because it is constantly being produced by cosmic rays. Tritium is also produced artificially in nuclear reactors. It is used as a radioactive tracer and as an ingredient of luminous paints and hydrogen bombs.