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Argon: the story on HearLore | HearLore
Argon
In 1894, two British scientists stared at a test tube containing a gas that refused to react with anything, and in doing so, they discovered a new element that would eventually be named for its laziness. Lord Rayleigh and Sir William Ramsay had been investigating a persistent discrepancy in the density of nitrogen. They found that nitrogen extracted from chemical compounds was 0.5% lighter than nitrogen drawn directly from the atmosphere. This tiny difference, barely perceptible to the naked eye, haunted them for months until they realized the air contained an invisible, unreactive component that had been hiding in plain sight. They isolated this gas by removing oxygen, carbon dioxide, and water from a sample of air, then passing an electric arc through the remaining mixture to eliminate any reactive gases. The residue was a colorless, odorless substance that did not burn, did not support combustion, and did not combine with any other element. They named it argon, derived from the Ancient Greek word meaning lazy or inactive, a fitting moniker for the first noble gas to be discovered. This discovery shattered the long-held belief that all gases in the air were already known, proving that the atmosphere still held secrets waiting to be found.
The Atomic Paradox
The atomic weight of argon presented a baffling puzzle to chemists in the late 19th century that threatened to upend the entire periodic table. When Henry Moseley later solved the problem, he revealed that the table was arranged by atomic number, not atomic weight, but the initial confusion was profound. Argon has an atomic weight greater than that of potassium, the element immediately following it, yet argon is chemically inert while potassium is a highly reactive metal. Dmitri Mendeleev had originally placed elements in order of atomic weight, so argon should have come before potassium, but its inertness suggested it belonged before the reactive alkali metals. This contradiction forced a rethinking of how elements were organized. The solution lay in the complete octet of electrons in argon's outer atomic shell, which made it stable and resistant to bonding. This electronic configuration meant that argon would not form compounds under normal conditions, unlike its neighbors. The discovery of argon also highlighted the existence of radiogenic argon-40, which is derived from the decay of potassium-40 in the Earth's crust. This process makes argon-40 the dominant isotope in Earth's atmosphere, comprising 99.6% of all argon found here, whereas in the universe, argon-36 is the most common isotope produced by stellar nucleosynthesis in supernovas. The abundance of argon-40 on Earth is so high that it makes argon the third most abundant gas in the atmosphere, accounting for 0.934% of the air we breathe, which is more than twice as abundant as water vapor and 23 times as abundant as carbon dioxide.
Lord Rayleigh and Sir William Ramsay discovered argon in 1894. They identified the element while investigating a discrepancy in the density of nitrogen extracted from chemical compounds versus nitrogen drawn from the atmosphere.
What is the atomic weight of argon and how does it compare to potassium?
Argon has an atomic weight greater than that of potassium, the element immediately following it in the periodic table. This contradiction forced chemists to rethink how elements were organized, leading to the realization that the table is arranged by atomic number rather than atomic weight.
How much argon is produced worldwide each year and how is it extracted?
Approximately 700,000 tonnes of argon are produced worldwide every year through the fractional distillation of liquid air in cryogenic air separation units. This process separates liquid nitrogen, argon, and liquid oxygen based on their different boiling points.
What are the primary uses of argon in modern industry and science?
Argon is used as an inert shielding gas in welding, for growing silicon and germanium crystals, and in neutrino experiments and dark matter searches. It also preserves national documents stored in argon-filled cases and extends the shelf life of food products.
Is argon dangerous to humans and what are the safety risks?
Argon is considered a dangerous asphyxiant because it is 38% more dense than air and displaces oxygen in closed areas. A 1994 incident in Alaska highlighted the risks of argon tank leakage in confined spaces where the gas is colorless, odorless, and tasteless.
When was the first argon compound formed and what is its stability?
Researchers at the University of Helsinki formed argon fluorohydride in August 2000 by shining ultraviolet light onto frozen argon containing hydrogen fluoride. This compound is stable up to 17 Kelvin or minus 256 degrees Celsius.
Today, approximately 700,000 tonnes of argon are produced worldwide every year, extracted industrially by the fractional distillation of liquid air in cryogenic air separation units. This process separates liquid nitrogen, which boils at 77.3 Kelvin, from argon, which boils at 87.3 Kelvin, and liquid oxygen, which boils at 90.2 Kelvin. The resulting gas is used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive. For example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning, and it is essential for arc welding techniques such as gas metal arc welding and gas tungsten arc welding. The gas is also used in the processing of titanium and other reactive elements, and for growing crystals of silicon and germanium. In the poultry industry, argon is used to asphyxiate birds, either for mass culling following disease outbreaks or as a means of slaughter more humane than electric stunning. Because argon is denser than air, it displaces oxygen close to the ground during inert gas asphyxiation, and its non-reactive nature makes it suitable in a food product, enhancing shelf life. The gas is also used for extinguishing fires where valuable equipment may be damaged by water or foam, and it is used as a propellant in aerosol cans. The bulk of its applications arise simply because it is inert and relatively cheap, making it the most plentiful noble gas produced by cryogenic air separation.
The Dark Matter Hunt
Liquid argon has become a critical target for neutrino experiments and direct dark matter searches, serving as the heart of some of the most advanced physics experiments on Earth. The interaction between hypothetical WIMPs and an argon nucleus produces scintillation light that is detected by photomultiplier tubes. Two-phase detectors containing argon gas are used to detect the ionized electrons produced during WIMP-nucleus scattering. Argon has a high scintillation light yield of about 51 photons per kiloelectron volt, is transparent to its own scintillation light, and is relatively easy to purify. Compared to xenon, argon is cheaper and has a distinct scintillation time profile, which allows the separation of electronic recoils from nuclear recoils. Dark-matter detectors currently operating with liquid argon include DarkSide, WArP, ArDM, microCLEAN, and DEAP. Neutrino experiments include ICARUS and MicroBooNE, both of which use high-purity liquid argon in a time projection chamber for fine-grained three-dimensional imaging of neutrino interactions. Most of the argon in Earth's atmosphere was produced by electron capture of long-lived potassium-40 present in natural potassium within Earth, but the argon-39 contamination in atmospheric argon is a significant challenge. The half-life of argon-39 is only 269 years, so underground argon, shielded by rock and water, has much less argon-39 contamination. This makes underground sources essential for sensitive experiments that require ultra-pure argon free from background radiation.
The Preservative Secret
Since 2002, the American National Archives has stored important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to inhibit their degradation. Argon is preferable to the helium that had been used in the preceding five decades because helium gas escapes through the intermolecular pores in most containers and must be regularly replaced. Argon is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents, and it has the European food additive code E938. Aerial oxidation, hydrolysis, and other chemical reactions that degrade the products are retarded or prevented entirely. High-purity chemicals and pharmaceuticals are sometimes packed and sealed in argon. In winemaking, argon is used in a variety of activities to provide a barrier against oxygen at the liquid surface, which can spoil wine by fueling both microbial metabolism and standard redox chemistry. Argon is also used as a preservative for such products as varnish, polyurethane, and paint, by displacing air to prepare a container for storage. The gas is used in laboratory equipment as the inert gas within Schlenk lines and gloveboxes, preferred to less expensive nitrogen in cases where nitrogen may react with the reagents or apparatus. It is also used as the carrier gas in gas chromatography and in electrospray ionization mass spectrometry, and it is the gas of choice for the plasma used in ICP spectroscopy. Argon gas is also commonly used for sputter deposition of thin films as in microelectronics and for wafer cleaning in microfabrication.
The Light and The Laser
Argon is used in incandescent lights to preserve the filaments at high temperature from oxidation, and it is used for the specific way it ionizes and emits light, such as in plasma globes and calorimetry in experimental particle physics. Gas-discharge lamps filled with pure argon provide lilac or violet light, while lamps with argon and some mercury produce blue light. Argon is also used for blue and green argon-ion lasers, which are used in surgery to weld arteries, destroy tumors, and correct eye defects. Blue argon lasers are used in surgery to weld arteries, destroy tumors, and correct eye defects, and the procedure carries a risk of producing gas embolism and has resulted in the death of at least one patient. Argon is used for thermal insulation in energy-efficient windows, and it is also used in technical scuba diving to inflate a dry suit because it is inert and has low thermal conductivity. The gas is used as a propellant in the development of the Variable Specific Impulse Magnetoplasma Rocket, and compressed argon gas is allowed to expand to cool the seeker heads of some versions of the AIM-9 Sidewinder missile and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure, and its properties make it ideal for these diverse applications. Argon has also been used experimentally to replace nitrogen in the breathing or decompression mix known as Argox, to speed the elimination of dissolved nitrogen from the blood.
The Atomic Ghost
Argon-36, in the form of argon hydride or argonium ions, has been detected in the interstellar medium associated with the Crab Nebula supernova, marking the first noble-gas molecule detected in outer space. This discovery confirmed that noble gases could form compounds under extreme conditions, even though they were previously thought to be completely inert. The first argon compound with tungsten pentacarbonyl, W(CO)5Ar, was isolated in 1975, but it was not widely recognized at that time. In August 2000, another argon compound, argon fluorohydride, was formed by researchers at the University of Helsinki by shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride with caesium iodide. This discovery caused the recognition that argon could form weakly bound compounds, even though it was not the first. The compound is stable up to 17 Kelvin, or minus 256 degrees Celsius. The metastable dication, which is valence-isoelectronic with carbonyl fluoride and phosgene, was observed in 2010. Solid argon hydride has the same crystal structure as the MgZn2 Laves phase and forms at pressures between 4.3 and 220 gigapascals, though Raman measurements suggest that the hydrogen molecules in the compound dissociate above 175 gigapascals. Theoretical calculations predict several more argon compounds that should be stable but have not yet been synthesized. These findings have expanded the understanding of chemical bonding and the limits of noble gas reactivity.
The Silent Danger
Although argon is non-toxic, it is 38% more dense than air and therefore considered a dangerous asphyxiant in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless, making it a silent killer in industrial and laboratory settings. A 1994 incident, in which a man was asphyxiated after entering an argon-filled section of oil pipe under construction in Alaska, highlights the dangers of argon tank leakage in confined spaces and emphasizes the need for proper use, storage, and handling. The gas displaces oxygen in enclosed spaces, and because it has no sensory warning, workers can enter an argon-rich environment without realizing the danger until it is too late. The triple point temperature of argon, 83.8058 Kelvin, is a defining fixed point in the International Temperature Scale of 1990, but this physical property does not mitigate the risks associated with its use. Argon is sometimes used as a doping agent to simulate hypoxic conditions, and in 2014, the World Anti-Doping Agency added argon and xenon to the list of prohibited substances and methods, although at this time there is no reliable test for abuse. The gas is used in cryosurgery procedures such as cryoablation to destroy tissue such as cancer cells, and it is used in a procedure called argon-enhanced coagulation, a form of argon plasma beam electrosurgery. The procedure carries a risk of producing gas embolism and has resulted in the death of at least one patient, underscoring the need for caution even in medical applications.