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or standard temperature and pressure,
standard conditions for measurement of the properties of matter. The standard temperature is the freezing point of pure water, 0°C; or 273.15°K;.
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; valence 0. Argon is a colorless, odorless, tasteless gas occurring in air (of which it constitutes 0.94% by volume) and in some volcanic gases. It is a member of Group 18 of the periodic tableperiodic table,
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, a group called the noble or inert gasesinert gas
or noble gas,
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 from the mistaken former belief that none of its members could form chemical compounds; in fact, other members of the group, e.g., krypton, xenon, and radon, do form compounds. Argon is prepared by fractional distillation of liquid air. Its extreme inertness has caused it to be substituted for nitrogen in electric light bulbs. It is mixed with neon in so-called neon signs (gas discharge tubes) to produce a green-to-blue glow. It is used as a protective atmosphere in arc welding, in the refining of reactive elements, and in the growing of crystals for use in semiconductor devices. Argon was first obtained by Lord Rayleigh and Sir William Ramsay in 1894. Previously Lord Rayleigh had noticed that a liter of supposedly pure nitrogen drawn from the air weighed more than a liter prepared from a nitrogen compound. This difference in weight led him to conclude that another gas was present in the supposedly pure nitrogen. Actually several unreactive gases were present; the first samples of "argon" also contained heliumhelium
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, neonneon
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, kryptonkrypton
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, and xenonxenon
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. Ramsay obtained pure argon later by evaporating it from liquid air.



Ar, a chemical element in Group VIII of the Mendeleev periodic system, belonging to the inert gases; atomic number 18, atomic mass 39.948. Under normal conditions argon is a colorless, odorless, and tasteless gas. It was discovered by J. Strutt (Lord Rayleigh), who in 1892 found an excess density of 0.0016 g/liter (at 0°C and 101,325 newtons per m2) of nitrogen in air as compared with the density of nitrogen in its compounds. In 1894, Rayleigh and W. Ramsay isolated from the nitrogen in air a gas that exhibited chemical inertness (Greek argos—idle). After the discovery of other inert gases, these were united as a separate zero group of the periodic system; now they are generally considered a major subgroup, Group VIII.

In nature argon exists only in the free state. The atmosphere contains 16 x 1012 tons of argon, the earth’s crust 0.165 x 1012 tons, and water 0.752 x 1012 tons. Volumetric concentration of argon in air is 0.93 percent. Atmospheric argon consists of three stable isotopes: Ar36 (0.337 percent), Ar38(0.063 percent), and Ar40 (99.600 percent). The predominance of the heavy isotope is due to its formation by the radioactive decay of natural potassium K40; as a result, the quantity of argon in the atmosphere is continually increasing. Because of the high amount of Ar40, the atomic mass of Ar is greater than is indicated in the Mendeleev table. Artificially obtained radioactive isotopes of argon used as radioactive tracers are made mainly from Ar37, which has a period of half-decay of 35.0 days. The density of argon (at 0°C and 101,325 N/m2) is 1.7839 kg/m3, the melting point is -189.3 °C, and the boiling point is -185.9°C. Under normal conditions one liter of water contains 51.9 cm3 of argon. In metals argon is practically insoluble. Molecular argon is monatomic. The primary ionization energy of argon (Ar° → Ar+) is great (15.755 electron volts); argon does not have an affinity to an electron. All attempts to obtain valent bonds using argon have been unsuccessful. Argon is capable of forming inclusion (clathrate) compounds with materials that have crystal lattice cells correspondingly close to the atomic diameter of argon (H2O, D2O, phenol, hydroquinone). The most extensively studied compound of argon is Ar-6H2O, which was first synthesized in 1896 by P. Vieillard through the crystallization of water in the presence of atmospheric argon (the pressure of argon is 15.5 MN/m2). The temperature of dissociation of Ar-6H2O at 101,325 N/m2 is 42.0°C. Argon with phenol gives the compound Ar.3C6H5OH. In inclusion compounds, for example in S02.6H2O, S02 can isomorphically replace argon (work of B. A. Nikitin and others), which makes it possible to synthesize Ar.6H2O at standard pressure.

In industry argon is obtained by a process of separation from air through supercooling. It is also possible to obtain argon from rectified gases of column synthesis of ammonia. Differentiating argon from other inert gases is most effectively accomplished through gas-chromatography methods. Argon is widely used in thermal processes for easy oxidation of metals. A protected atmosphere of argon can be used, for example, to conduct welding and cutting of various rare and nonferrous metals and smelting of Ti, W, Zr, and other metals. Crystalline semiconducting materials are also formed using an argon atmosphere. Argon is used to fill electric bulbs (argon lowers the speed of tungsten vaporization and increases luminescence). Argon tubes are used for advertising (deep blue-azure color). Determination of the ratio Ar40:K40 is used as a basis in one of the methods of computing the age of minerals. Radioactive argon is sometimes used for the control of ventilation systems.


Fastovskii, V. G., A. E. Rovinskii, and Iu. V. Petrovskii. Inertnye gazy. Moscow, 1964.
Golovko, G. A. Apparaty i ustanovki dlia proizvodstva argona. Moscow-Leningrad, 1965.
Finkel’shtein, D. N. Inertnye gazy. Moscow, 1961.
Berdonosov, S. S. Inertnye gazy vchera i segodnia. Moscow, 1966.



A chemical element, symbol Ar, atomic number 18, atomic weight 39.998.


an extremely unreactive colourless odourless element of the rare gas series that forms almost 1 per cent (by volume) of the atmosphere. It is used in electric lights. Symbol: Ar; atomic no.: 18; atomic wt.: 39.948; density: 1.7837 kg/m3; freezing pt.: --189.3°C; boiling pt.: --185.9°C