Argon

• 1. Introduction

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  Argon makes up nearly one percent of the air you are breathing right now. Symbol Ar, atomic number 18, it sits quietly in group 18 of the periodic table alongside the other noble gases. It is the third most abundant gas in Earth's atmosphere, at 0.934 percent by volume. That puts it ahead of water vapor on average, far ahead of carbon dioxide, and more than five hundred times more abundant than neon. Yet for most of human history, no one knew it was there. How does a gas that fills roughly one in every hundred breaths go unnoticed for so long? And once found, what does a substance famous for doing almost nothing actually turn out to be useful for?

• 2. The Name and Its Meaning

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  The word argon comes from the Ancient Greek ἀργόν, the neuter singular form of ἀργός, meaning lazy or inactive. The people who named it chose that word deliberately. An element that refuses to react with almost anything else struck its discoverers as deserving a name that announced its indifference to chemistry. That indifference comes down to structure. Argon carries a complete octet of electrons in its outer shell, eight electrons filling both the s and p subshells. A full valence shell leaves no appetite for bonding. Before 1962, argon and its noble-gas relatives were treated as chemically inert by definition, incapable of forming any compound at all. That assumption held for decades, and it shaped how chemists thought about the entire right-hand column of the periodic table.

• 3. Discovery and the Puzzle It Created

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  Henry Cavendish suspected as early as 1785 that air contained an unreactive gas hiding within it, but he could not isolate what it was. The proof came in 1894, when Lord Rayleigh and Sir William Ramsay at University College London removed oxygen, carbon dioxide, water, and nitrogen from a clean air sample and found a residue that would not react with anything. Before that isolation, they had already noticed something odd: nitrogen extracted from chemical compounds was consistently about 0.5 percent lighter than nitrogen drawn from the atmosphere. That small but persistent difference held their attention for many months and pointed toward a hidden ingredient. H. F. Newall and W. N. Hartley had each independently spotted new lines in the emission spectrum of air in 1882, lines that matched no known element, though neither team connected those lines to a discrete gas at the time. The discovery of argon also created an awkward problem for the periodic table. Dmitri Mendeleev had ordered his table by atomic weight, but argon's atomic weight turned out to be greater than that of potassium, the element immediately after it. Argon's chemical inertness clearly placed it before the reactive alkali metal, yet the weight ordering said otherwise. Henry Moseley later resolved the puzzle by showing that the true organizing principle is atomic number, not atomic weight.

• 4. Where Argon Comes From

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  Nearly all argon in Earth's atmosphere is radiogenic argon-40, meaning it was not primordial but was produced over geological time by the radioactive decay of potassium-40 inside Earth's crust. That origin sets terrestrial argon apart from what exists elsewhere in the universe. In the cosmos broadly, argon-36 dominates because it is the isotope most readily generated by stellar nucleosynthesis inside supernovas. Solar wind measurements show that the sun's argon is 84.6 percent argon-36. The atmospheres of the outer planets carry the three main isotopes in a ratio of 8400 to 1600 to 1 for argon-36, argon-38, and argon-40, a profile entirely unlike Earth's. Mars, Mercury, and Titan, the largest moon of Saturn, all have argon in their atmospheres, predominantly as argon-40. On Earth, about 700,000 tonnes of argon are produced industrially every year, extracted from liquid air by cryogenic fractional distillation, a process that separates liquid nitrogen boiling at 77.3 K from argon boiling at 87.3 K and liquid oxygen boiling at 90.2 K.

• 5. Breaking the Inert Label

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  Argon's reputation for total chemical passivity turned out to be only approximately true. The first argon compound with a metal, a complex with tungsten pentacarbonyl written as W(CO)5Ar, was isolated in 1975, though it attracted little notice at the time. The moment that changed the field came in August 2000, when researchers at the University of Helsinki formed argon fluorohydride, HArF, by shining ultraviolet light onto frozen argon that contained a small amount of hydrogen fluoride with caesium iodide. The compound is stable only below 17 kelvins, which is roughly negative 256 degrees Celsius. Argon-36, in the form of argonium ions, has also been detected in the interstellar medium associated with the Crab Nebula supernova, making it the first noble-gas molecule ever detected in outer space. Solid argon hydride, written as Ar(H2)2, shares the same crystal structure as the MgZn2 Laves phase and forms under pressures between 4.3 and 220 GPa. The triple point temperature of argon, 83.8058 K, is precise enough that it serves as a defining fixed point in the International Temperature Scale of 1990.

• 6. Industrial and Everyday Uses

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  Argon's practical value rests almost entirely on two facts: it will not react with most materials even at high temperatures, and it is cheap because it emerges as a byproduct when liquid oxygen and liquid nitrogen are produced from air. Welding operations rely on argon as a shielding gas in processes such as gas metal arc welding and gas tungsten arc welding, where oxygen or nitrogen in the atmosphere would otherwise cause defects in the weld. Graphite electric furnaces use an argon atmosphere to keep the graphite from burning. Growing crystals of silicon and germanium also requires an argon blanket. Beyond the foundry, argon fills incandescent light bulbs to protect the filament from oxidation at high temperature. Pure argon in a gas-discharge lamp produces a lilac or violet light; mixed with mercury, it produces blue. Argon-ion lasers emit blue and green light and have found use in surgery, where they are used to weld arteries, destroy tumors, and correct eye defects. In the poultry industry, argon is used to asphyxiate birds either during disease culling or as a more humane alternative to electric stunning, because it is denser than air and displaces oxygen close to the ground without leaving chemical residues in the carcass.

• 7. Preservation and Protection

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  Since 2002, the American National Archives has stored documents including the Declaration of Independence and the Constitution in argon-filled cases. Argon replaced helium, which had been used for the preceding five decades but tends to escape through the intermolecular pores of most containers and must be regularly replenished. Argon carries the European food additive code E938 and is used in food packaging to displace oxygen and slow oxidation, hydrolysis, and other degradation reactions. Winemakers use it at the liquid surface to block the oxygen contact that feeds both microbial metabolism and standard redox chemistry. High-purity pharmaceuticals and chemicals are sometimes sealed under argon as well. In the laboratory, argon fills Schlenk lines and gloveboxes where even nitrogen might react with sensitive reagents. Argon is also used for the sputter coating of specimens in scanning electron microscopy and for sputter deposition of thin films in microelectronics fabrication.

• 8. Science at the Frontier

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  Liquid argon has become a preferred target material for two of the most ambitious experiments in physics: neutrino detection and direct dark matter searches. When a hypothetical WIMP particle collides with an argon nucleus, it produces scintillation light that photomultiplier tubes can detect. Argon emits about 51 photons per keV of deposited energy, is transparent to its own scintillation light, and is easier to purify than xenon. Operating dark matter detectors using liquid argon include DarkSide, WArP, ArDM, microCLEAN, and DEAP. Neutrino experiments ICARUS and MicroBooNE both use high-purity liquid argon in time projection chambers that create fine-grained three-dimensional images of neutrino interactions. Underground argon, shielded by rock and water from the cosmic-ray reactions that continually produce radioactive argon-39 in the atmosphere, carries significantly less radioactive contamination, making it especially valuable for experiments that need the quietest possible target. Argon-39 itself, with a half-life of 269 years, is used for dating ice cores and groundwater. The same radioactive relationship between potassium-40 and argon-40 underpins potassium-argon dating, a method applied to sedimentary, metamorphic, and igneous rocks across geology.