Protium. H; at. wt 1.0079; at. no. 1; valence 1; elemental state H2. Isotopes: 1 (protium 99.9844%); 2 (deuterium 0.0156%); 3 (tritium, traces only). The most abundant element in the known universe. Occurence in the earth's atmosphere 0.00005% H2. First recognized as an element by Cavendish in 1766; named by Lavoisier. Obtained by passing H2O vapors over heated iron; by electrolysis of water or by action of HCl or H2SO4 on Fe or Zn; by hydrolysis of metal hydrides. Produced industrially by electrolysis; from methane or coke and steam. Reviews: Nouveau Traité de Chimie Minérale vol. 1, P. Pascal, Ed. (Masson, Paris, 1956) pp 565-675; Mackey in Comprehensive Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1-76; B. G. Mandelik, D. S. Newsome in Kirk-Othmer Encyclopedia of Chemical Technology vol. 12, (Wiley-Interscience, New York, 3rd ed., 1980) pp 938-982. See also Deuterium and Tritium.

Colorless, odorless, tasteless gas; flammable or explosive when mixed with air, oxygen, chlorine, etc. mp -259.2° (13.96°K) at 54 mm (triple point). bp -252.77° (20.39°K). dgas 0.069 (air = 1); dliq 0.0700 (at bp); dsol 0.0763 (13°K). A liter of gas at 0° weighs 0.08987 g. Crit. temp -239.9°; crit press. 12.8 atm. Sol in about 50 vols of water at 0°.

Caution: No specific toxic action. In high concns can act as a simple asphyxiant.

USE: In oxy-hydrogen blowpipe (welding) and limelight; autogenous welding of steel and other metals; manuf ammonia, synthetic methanol, HCl; hydrogenation of oils, fats, naphthalene, phenol; in ballons and airships; manuf tungsten. In thermonuclear reactions: the hydrogen atom has a single peripheral electron (1s). It ionizes to form protons, deuterons (D) or tritons (T). The ionization potential of the H atom is 13.59 eV. Accelerated protons bring about extremely varied nuclear reactions. Liq hydrogen used in bubble chambers to study subatomic particles; as a coolant.