H

The name hydrogen is taken from the Greek hydro + genes, or "water generator". The gas is colorless, odorless and tasteless and is the most abundant element in the known universe. It is also the lightest (in terms of atomic mass) and the simplest, having only one proton and one electron (and no neutrons in its most common isotope).

First isolated and identified as an element by Cavendish in 1766, hydrogen was believed to be many different things. Cavendish himself thought that it was "inflammable air from metals", owing to its production by the action of acids on metals. Others thought it was pure phlogiston because of its flammability. Hydrogen is among the ten most abundant elements on the planet, but very little is found in elemental form due to its low density and reactivity. Much of the terrestrial hydrogen is locked up in water molecules and organic compounds like hydrocarbons.

The vast majority of hydrogen produced industrially today is made either from treatment of methane gas with steam or in the production of "water gas" from the reaction of coal with steam. Most of this hydrogen is used in the Haber process to manufacture ammonia. The element is also used for hydrogenating vegetable oils, turning them into margarine and shortening, and some is used for liquid rocket fuel.

There are two important isotopes of hydrogen. Deuterium (²H) has an abundance of 0.015% of terrestrial hydrogen and the nucleus of the isotope contains one neutron. Sometimes called heavy hydrogen, deuterium is separated by the fractional distillation of liquid hydrogen but it can also be produced by the prolonged electrolysis of ordinary water. Approximately 100,000 gallons of water will produce a single gallon of D₂O, "heavy water". This special kind of water is used as a moderator in some fission power reactors. Deuterium fuel is used in experimental fusion reactors.

Tritium (³H) contains two neutrons in its nucleus and is radioactive. It is very rare (about 1 in every 10¹⁸ atoms) and is formed in the environment by cosmic ray bombardment. Most tritium is manufactured by bombarding Li with neutrons. Tritium is used in thermonuclear weapons and experimental fusion reactors.