Argon, derived from the Greek word argos, meaning inactive, is the third most prevalent gas in the atmosphere. It has an atomic mass of 39.948 and an atomic radius of 0.88 Angstrom.

Argon was the first noble gas to be discovered. The discovery by Sir William Ramsay in 1894 was the result of a discrepancy between the density of supposedly pure nitrogen, prepared from air, and actually pure nitrogen prepared from ammonium nitrate.

Argon is a monatomic gas, which emits a blue colour in a vacuum tube. It has a melting point of 83.81K and a boiling point of 87.30K It is abundant in air at 9300 parts per million, constituting 0.93 percent of the atmosphere. It is produced in the earth's lithosphere by the radioactive decay of potassium. Argon has a cubic-faced centred crystal structure.

Argon is produced by the fractional distillation of liquid air. It is used in large quantities to fill electric light bulbs as it provides an inert atmosphere. It is a poor conductor of heat and so cools the filament less. Because of its high specific heat compared with air, argon is used to create protective barriers between molten metals and oxygen contained in the air. This reduces losses by oxidation. It is also used for arc welding, steel making, lasers, and fluorescent lamps.

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