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Argon is a chemical element with the symbol Ar and atomic number 18. It is a noble gas, colorless, odorless, and chemically inert under standard conditions. It is the third most abundant gas in Earth's atmosphere, making up about 0.93% by volume.

Argon is a monatomic gas with a boiling point of −185.8°C and a density of 1.784 g/L at standard temperature and pressure. It forms very few compounds; the known argon compounds are highly unstable and require extreme conditions to form. Because of its inertness, argon is widely used to provide nonreactive environments in various industrial processes.

Production of argon occurs primarily through the fractional distillation of liquid air in large air-separation plants. It is harvested as a byproduct of extracting nitrogen and oxygen from the atmosphere. The primary applications of argon include shielding gas in arc welding and metal fabrication, inert atmospheres for growing silicon and other crystals, and filling incandescent and fluorescent lighting to prevent filament oxidation. It is also used in certain specialized optical and laser systems.

Isotopes and dating: Natural argon consists mainly of the stable 40Ar, with small fractions of 36Ar and 38Ar, and trace amounts of 39Ar. The 40Ar isotope is produced by the radioactive decay of 40K and is central to argon-argon dating, a radiometric dating technique used in geology and archaeology.

History and name: Argon was discovered in 1894 by Lord Rayleigh and Sir William Ramsay while studying air gases. The name derives from the Greek argos, meaning idle or inactive, reflecting its inertness.