4792. Hydrogen

Nomenclature

CAS number: 1333-74-0
Protium.

Description and references

H; at. wt 1.00794; at. no. 1; valence 1. Group IA (1). Elemental state: H2. Exists in two forms, distinguished by the nuclear spins of the atoms: ortho has parallel spins, para has antiparallel spins. Normal hydrogen is a 3:1 equilibrium ratio of ortho to para at rm temp. Naturally occurring isotopes: 1 (protium 99.985%); 2 (deuterium 0.015%); 3 (tritium, traces only). The most abundant element in the known universe. Occurrence 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 steam reforming, partial oxidation, coal gasification and water electrolysis. Reviews: Nouveau Traité de Chimie Minérale vol. 1, P. Pascal, Ed. (Masson, Paris, 1956) pp 565-675; Mackay in Comprehensive Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1-76; Chemistry of the Elements N. N. Greenwood, A. Earnshaw, Eds. (Pergamon Press, New York, 1984) pp 38-74; T. A. Czuppon et al. in Kirk-Othmer Encyclopedia of Chemical Technology vol. 13 (Wiley-Interscience, New York, 4th ed., 1995) pp 838-894. See also Deuterium and Tritium.

Properties

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 the 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°. Ionization potential of H atom 13.59 eV.

Caution

Can act as an asphyxiant by displacing air. See: Matheson Gas Data Book (Matheson, 6th ed., Lyndhurst, NJ, 1980) pp 366-371.

Use

In oxy-hydrogen blowpipe (welding) and limelight; autogenous welding of steel and other metals; manuf ammonia, synthetic methanol, HCl, NH3; hydrogenation of oils, fats, naphthalene, phenol; in balloons and airships; in metallurgy to reduce oxides to metals; in petroleum refining; in thermonuclear reactions (ionizes to form protons, deuterons (D) or tritons (T)). Liq hydrogen used in bubble chambers to study subatomic particles; as a coolant.