Argon. Ar; at. wt. 39.948; at. no. 18. Three stable isotopes: 36 (0.337%); 38 (0.063%); 40 (99.600%); artifical, radioactive isotopes: 33; 35; 37; 39; 41; 42. Abundance in earth's crust: 4 x 104%; concentration in the atmosphere: 0.93% by vol; cosmic abundance: ~1.5 x 105 atom/106 atoms of Si. Elemental, monoatomic, gaseous constituent of air, discovered by Rayleigh and Ramsay in 1894. Although molecular ions, hydrates and clathrates of argon have been observed, it should be considered a "noble", chemically inerts gas, due to its electronic structure. The outer p subshell is entirely filled: 1s22s22p63s23p6. Obtained commercially during liquid-air manufacture. A radioactive isotope 41Ar (110 min, b-) is found in the air surrounding atomic reactors, when n,g reaction is possible. Monograph: Argon, Helium and the Rare Gases, vol. 1, 2, G.A. Cook, Ed. (Inerscience, New York, 1961) 818 pp. Review: Cockett, Smith, "The Monatomic Gases" in Comprehensive Inorganic Chemistry, vol. 1, J. C. Bailar, Jr. et al., Eds (Pergamon Press, Oxford, 1973) pp 139-211.

Colorless, odorless, inert gas. d (gas) 1.784 g/l. bp -185.86 (87.29K); crit temp -122.3; critical press 48.3 atm. Crystallizes in a face-centered cubic lattice; triple pt -189.37 (83.78K). d (solid at triple pt) 1.623. Atomic radius 1.92 x 10-8 cm. Soly of gas in water at 20: 33.6 cc/kg water. Also sol in organic liquids.

USE: In fluorescent tubes analogous to neon lights, but produces a bluish-purplish light; in rectifier tubes; in thermometers above mercury; in lasers; wherever an inert atmosphere is desired and the much cheaper nitrogen cannot be used; in ionization chambers and particle counters. The isotope 40Ar is always found in minerals contg potassium, since it is a product of 40K decay. Measuring the amount of 40Ar and 40K can be used for determining the geologic age of minerals and meteors. Human Toxicity: Simple asphyxiant.
Ref.: 808, pp 123-124.
The Merck Index - Eleventh Edition.
Copyright 1989 by Merck & Co., Inc.