ARGON, Ar (from the Greek word "argon" (inactive) * EN: argon; DE: Argon; FR: argon; ES: argon; RU: аргон), is the chemical element of the main subgroup of the Group VIII of the Periodic system by Mendeleev, it belongs to the inert gases, the atomic number is 18, the atomic mass is 39.948. It consists of the three stable isotopes, the major one is 40Ar (99.600%). It has been isolated from the air in 1894 by the British scientists J.Rayleigh and W.Ramsay.

Argon exists within nature only in the free form. Argon under the ordinary conditions is the gas without colour, smell, and taste. Solid argon crystallizes into the cubic crystal systems. The density of argon is 1.78 kilograms per cubic metre, the temperature of the melting point is minus 189.3 degrees Celsius, the temperature of the boiling point is minus 185.9 degrees Celsius, the critical pressure is 48 megapascals, the critical temperature is minus 122.44 degrees Celsius. The first ionization potential is 15.69 electron-volt. The atomic radius is 0.188 nanometres (1.88 angstroms).

The chemical compounds have not been obtained (only the inclusion compounds are known). There may be dissolved under the normal conditions 51.9 cubic centimetres of argon within 1 litre of the distilled water. It forms crystalline hydrates of the Ar • 6H2O type. The Clarke (weight) number within the Earth's crust is 4 • 10-4; the content within atmosphere is 0.9325% by volume (6.5 • 1016 kilograms), within the eruptive rocks is 2.2 • 10-5 cubic centimetres per gram, within the oceanic water is 0.336 cubic centimetres per litre. There has been produced 5.3 • 1019 kilograms of 40Ar within the mantle, the average rate of accumulation of 40Ar within the Earth's crust is 2 • 107 kilograms per year.

The atoms of argon migrate from minerals along the dislocations within the crystal structure into the zones of violations of the crystal lattice, and then arrive along the microcracks and pores into the formation (layer) waters, oil and gas reservoirs. There is based on the measurement of the ratio of the 40Ar/40K contents within the potassium-containing minerals the method for the determination of the age of the geological objects. They determine with the argon method the age of the eruptive (according to micas, amphiboles), sedimentary (according to glauconites, sylvites), metamorphized rocks, for which the age of the metamorphism is also given with the known approximation. There has been developed the activation method of the dating, which is based on the measurement of the ratio of 40Ar/39Ar. They obtain argon within the industry during the process of air separation during the deep cooling. The obtainment of argon from the purge gases of the ammonia synthesis columns is possible. The separation of argon from the other inert gases is performed most completely with the gas chromatography method.

Argon is used during the thermal processing of the easily oxidizable metals. They perform within the protective atmosphere of argon the welding and cutting of the rare and non-ferrous metals, the smelting of the titanium, tungsten, zirconium, and so on, they grow the crystals of the semiconductor materials. They use the radioactive isotope (37Ar) for the monitoring of the ventilation systems.