A chemical element, Ar, atomic number 18, and atomic weight 39.948. Argon is the third member of group 18 in the periodic table. The gaseous elements in this group are called the noble, inert, or rare gases, although argon is not actually rare. The Earth's atmosphere is the only natural argon source; however, traces of this gas are found in minerals and meteorites. Argon constitutes 0.934% by volume of the Earth's atmosphere. Of this argon, 99.6% is the argon-40 isotope; the remainder is argon-36 and argon-38. There is good evidence that all the argon-40 in the air was produced by the radioactive decay of the radioisotope potassium-40.
Argon is colorless, odorless, and tasteless. The element is a gas under ordinary conditions, but it can be liquefied and solidified readily. Some salient properties of the gas are listed in the table. Argon does not form any chemical compounds in the ordinary sense of the word, although it does form some weakly bonded clathrate compounds with water, hydroquinone, and phenol. There is one atom in each molecule of gaseous argon.
The oldest large-scale use for argon is in filling electric light bulbs. Welding and cutting metal consumes the largest amount of argon. Metallurgical processing constitutes the most rapidly growing application. Argon and argon-krypton mixtures are used, along with a little mercury vapor, to fill fluorescent lamps. Argon mixed with a little neon is used to fill luminous electric-discharge tubes employed in advertising signs (similar to neon signs) when a blue or green color is desired instead of the red color of neon. Argon is also used in gas-filled thyratrons, Geiger-Muller radiation counters, ionization chambers which measure cosmic radiation, and electron tubes of various kinds. Argon atmospheres are used in dry boxes during manipulation of very reactive chemicals in the laboratory and in sealed-package shipments of such materials.
Most argon is produced in air-separation plants. Air is liquefied and subjected to fractional distillation. Because the boiling point of argon is between that of nitrogen and oxygen, an argon-rich mixture can be taken from a tray near the center of the upper distillation column. The argon-rich mixture is further distilled and then warmed and catalytically burned with hydrogen to remove oxygen. A final distillation removes hydrogen and nitrogen, yielding a very high-purity argon containing only a few parts per million of impurities.