Kaplan, Louis Argonne National Laboratory, Argonne, Illinois.
Last reviewed:July 2018
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A chemical element, H, atomic number 1, atomic weight 1.008. Hydrogen is the first chemical element in the periodic table. Under ordinary conditions it is a colorless, odorless, tasteless gas composed of diatomic molecules, H2. The hydrogen atom, symbol H, consists of a nucleus of unit positive charge and a single electron. It has atomic number 1 and an atomic weight of 1.008. The element is a major constituent of water and all organic matter, and is widely distributed not only on the Earth but throughout the universe. There are three isotopes of hydrogen: protium, mass 1, makes up 99.98% of the natural element; deuterium, mass 2, makes up about 0.02%; and tritium, mass 3, occurs in extremely small amounts in nature but may be produced artificially by various nuclear reactions. See also: Deuterium; Isotope; Periodic table; Tritium; Universe
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