Entry 18

Argon (chemical symbol Ar, atomic number 18) is a colourless, odourless noble gas that occupies the third‑largest share of Earth’s atmosphere at roughly 0.93 % by volume. Its complete octet of electrons renders it chemically inert under ordinary conditions, a property that has made argon indispensable for creating protective atmospheres in a wide range of high‑precision technologies. Discovered in 1894 by Lord Rayleigh and Sir William Ramsay, argon was the first noble gas identified on the planet, opening a new chapter in the periodic table and in atmospheric science.

Historical Discovery

Lord Rayleigh’s 1894 experiments on the density of nitrogen revealed a slight excess that could not be accounted for by known gases. Collaborating with William Ramsay, who had already isolated helium from uranium ore, the pair isolated the residual gas by removing oxygen and nitrogen from air through chemical absorption and fractional distillation. In December 1894 they announced the existence of a new element, naming it “argon” from the Greek ἀργός (“inactive”), reflecting its lack of chemical reactivity.

Physical and Chemical Properties

Argon’s atomic mass is 39.948 u, and its electron configuration is [Ne] 3s² 3p⁶, giving it a full valence shell. At standard temperature and pressure it exists as a monatomic gas with a boiling point of −185.8 °C and a melting point of −189.3 °C, making it one of the coldest liquids obtainable by simple cooling. The element has three stable isotopes—⁴⁰Ar (99.6 %), ³⁸Ar (0.06 %) and ³⁶Ar (0.34 %)—the radiogenic ⁴⁰Ar originates from the β‑decay of ⁴⁰K in the Earth’s crust, a fact that underpins potassium‑argon dating of volcanic rocks.

Industrial Production and Global Supply

Modern argon is obtained almost exclusively by fractional distillation of liquid air, a process that separates gases according to their distinct boiling points. The global production capacity exceeded 1.5 million tonnes in 2023, with major facilities located in the United States, China, and Russia. Purity levels of 99.999 % (five‑nines) are standard for most industrial uses, while ultra‑high‑purity grades (≥ 99.9999 %) are produced for semiconductor manufacturing and scientific research.

Major Applications

In welding, argon serves as a shielding gas for both TIG (gas‑metal‑arc) and MIG (metal‑inert‑gas) processes, displacing atmospheric oxygen and nitrogen to prevent oxidation of the molten metal; typical flow rates range from 10 to 20 L min⁻¹. The lighting industry fills incandescent bulbs and fluorescent tubes with argon to inhibit filament evaporation and to sustain the electric discharge, respectively, with a typical bulb containing 0.5 mL of the gas at atmospheric pressure. Argon’s inertness also makes it the preferred carrier gas in gas‑chromatography, a plasma medium in metal‑cutting lasers, and the detection medium in liquid‑argon time‑projection chambers used in neutrino experiments such as DUNE.

Scientific and Environmental Significance

The isotopic composition of atmospheric argon provides a baseline for geochemical studies; deviations in the ³⁸Ar/⁴⁰Ar ratio signal volcanic outgassing or mantle contributions. Potassium‑argon dating, first refined in the 1950s, relies on the predictable accumulation of ⁴⁰Ar to determine ages of rocks older than 100 ka, a technique that has calibrated the timescale of the Earth’s early crust. Moreover, argon’s high thermal conductivity (≈ 0.017 W m⁻¹ K⁻¹) and low specific heat make it useful in cryogenic applications, including the preservation of historic documents and the controlled atmosphere storage of fine wines, where oxygen‑free conditions extend shelf life.