Hydrogen

Hydrogen, chemical element, symbol H; for physical constants see Periodic Table. Hydrogen was prepared for many years before it was recognized as a distinct substance by Henry Cavendish in 1766. It is the most abundant element in the universe, making up about three quarters of the mass of the universe. On earth hydrogen occurs chiefly in combination with oxygen in water. Hydrogen is prepared commercially by the action of steam on heated carbon or iron. Hydrogen is a diatomic, colorless, explosively reactive gas, the lightest of all gases. Production of hydrogen in the United States alone amounts to hundreds of millions of cubic feet per day. Hydrogen gas is a mixture of 2 kinds of molecules, known as ortho- and para-hydrogen, which differ from one another by the spins of their electrons and nuclei. Hydrogen is used in nitrogen fixation, as rocket fuel, in reducing metallic ores, for welding, as heavy water in nuclear reactors, and as a neutron moderator. Liquid hydrogen is used in cryogenics.