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Argon is a ; it has symbol Ar and 18. It is in group 18 of the and is a .In older versions of the periodic table, the noble gases were identified as Group VIIIA or as Group 0. See Group (periodic table). Argon is the third most abundant in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as (400 ppmv), and more than 500 times as abundant as (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Nearly all argon in Earth's atmosphere is argon-40, derived from the decay of potassium-40 in Earth's crust. In the universe, argon-36 is by far the most common argon , as it is the most easily produced by stellar in .

The name "argon" is derived from the word ἀργόν, neuter singular form of ἀργός meaning 'lazy' or 'inactive', as a reference to the fact that the element undergoes almost no chemical reactions. The complete (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its temperature of 83.8058  is a defining fixed point in the International Temperature Scale of 1990.

Argon is extracted industrially by the fractional distillation of . It is mostly used as an in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in electric furnaces to prevent the graphite from burning. It is also used in incandescent and fluorescent lighting, and other gas-discharge tubes. It makes a distinctive blue-green gas laser. It is also used in fluorescent glow starters.


Characteristics
Argon has approximately the same in water as and is 2.5 times more soluble in water than . Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas. Argon is chemically under most conditions and forms no confirmed stable compounds at room temperature.

Although argon is a , it can form some compounds under various extreme conditions. Argon fluorohydride (HArF), a compound of argon with and that is stable below , has been demonstrated. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form with water when atoms of argon are trapped in a lattice of water molecules. , such as , and , such as ArF, have been demonstrated. Theoretical calculation predicts several more that should be stable but have not yet been synthesized.


History
Argon ( ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive") is named in reference to its chemical inactivity. This chemical property of this first to be discovered impressed the namers. An unreactive gas was suspected to be a component of air by in 1785.

Argon was first isolated from air in 1894 by Lord Rayleigh and Sir at University College London by removing , , water, and from a sample of clean air. They first accomplished this by replicating an experiment of 's. They trapped a mixture of atmospheric air with additional oxygen in a test-tube (A) upside-down over a large quantity of dilute solution (B), which in Cavendish's original experiment was potassium hydroxide, and conveyed a current through wires insulated by U-shaped glass tubes (CC) which sealed around the platinum wire electrodes, leaving the ends of the wires (DD) exposed to the gas and insulated from the alkali solution. The arc was powered by a battery of five and a of medium size. The alkali absorbed the oxides of nitrogen produced by the arc and also carbon dioxide. They operated the arc until no more reduction of volume of the gas could be seen for at least an hour or two and the spectral lines of nitrogen disappeared when the gas was examined. The remaining oxygen was reacted with alkaline pyrogallate to leave behind an apparently non-reactive gas which they called argon.

Before isolating the gas, they had determined that nitrogen produced from chemical compounds was 0.5% lighter than nitrogen from the atmosphere. The difference was slight, but it was important enough to attract their attention for many months. They concluded that there was another gas in the air mixed in with the nitrogen. Argon was also encountered in 1882 through independent research of H. F. Newall and W. N. Hartley.

(2025). 9780198503408, Oxford University Press. .
Each observed new lines in the emission spectrum of air that did not match known elements.

Prior to 1957, the symbol for argon was "A". This was changed to Ar after the International Union of Pure and Applied Chemistry published the work Nomenclature of Inorganic Chemistry in 1957.


Occurrence
Argon constitutes 0.934% by volume and 1.288% by mass of Earth's atmosphere. Air is the primary industrial source of purified argon products. Argon is isolated from air by fractionation, most commonly by fractional distillation, a process that also produces purified , , , and . Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.
(2025). 9780199605637, Oxford University Press. .


Isotopes
The main of argon found on Earth are (99.6%), (0.34%), and (0.06%). Naturally occurring , with a of 1.25 years, decays to stable (11.2%) by or positron emission, and also to stable (88.8%) by . These properties and ratios are used to determine the age of rocks by K–Ar dating.

In Earth's atmosphere, is made by activity, primarily by neutron capture of followed by two-neutron emission. In the subsurface environment, it is also produced through by , followed by proton emission. is created from the by followed by an emission as a result of subsurface . It has a half-life of 35 days.

Between locations in the , the isotopic composition of argon varies greatly. Where the major source of argon is the decay of in rocks, will be the dominant isotope, as it is on Earth. Argon produced directly by stellar nucleosynthesis is dominated by the nuclide . Correspondingly, solar argon contains 84.6% (according to measurements), and the ratio of the three isotopes 36Ar : 38Ar : 40Ar in the atmospheres of the outer planets is 8400 : 1600 : 1. This contrasts with the low abundance of primordial in Earth's atmosphere, which is only 31.5 ppmv (= 9340 ppmv × 0.337%), comparable with that of neon (18.18 ppmv) on Earth and with interplanetary gasses, measured by probes.

The atmospheres of Mars, Mercury and Titan (the largest moon of ) contain argon, predominantly as .

The predominance of is the reason the standard atomic weight of terrestrial argon is greater than that of the next element, , a fact that was puzzling when argon was discovered. positioned the elements on his in order of atomic weight, but the inertness of argon suggested a placement before the reactive . later solved this problem by showing that the periodic table is actually arranged in order of (see History of the periodic table).


Compounds
Argon's complete octet of indicates full s and p subshells. This full makes argon very stable and extremely resistant to bonding with other elements. Before 1962, argon and the other noble gases were considered to be chemically inert and unable to form compounds; however, compounds of the heavier noble gases have since been synthesized. The first argon compound with tungsten pentacarbonyl, W(CO)5Ar, was isolated in 1975. However, it was not widely recognised at that time. In August 2000, another argon compound, argon fluorohydride (HArF), was formed by researchers at the University of Helsinki, by shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride with . This discovery caused the recognition that argon could form weakly bound compounds, even though it was not the first. It is stable up to 17 kelvins (−256 °C). The dication, which is valence- with carbonyl fluoride and , was observed in 2010. Argon-36, in the form of argon hydride () ions, has been detected in interstellar medium associated with the ; this was the first detected in .

Solid argon (Ar(H2)2) has the same crystal structure as the MgZn2 . It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the H2 molecules in Ar(H2)2 dissociate above 175 GPa.


Production
Argon is extracted industrially by the fractional distillation of in a unit; a process that separates , which boils at 77.3 K, from argon, which boils at 87.3 K, and , which boils at 90.2 K. About 700,000 of argon are produced worldwide every year.


Applications
Argon has several desirable properties:
  • Argon is a chemically .
  • Argon is the cheapest alternative when is not sufficiently inert.
  • Argon has low thermal conductivity.
  • Argon has electronic properties (ionization and/or the emission spectrum) desirable for some applications.

Other would be equally suitable for most of these applications, but argon is by far the cheapest. It is inexpensive, since it occurs naturally in air and is readily obtained as a byproduct of in the production of and : the primary constituents of air are used on a large industrial scale. The other noble gases (except ) are produced this way as well, but argon is the most plentiful by far. The bulk of its applications arise simply because it is inert and relatively cheap.


Industrial processes
Argon is used in some high-temperature industrial processes where ordinarily non-reactive substances become reactive. For example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning.

For some of these processes, the presence of nitrogen or oxygen gases might cause defects within the material. Argon is used in some types of such as gas metal arc welding and gas tungsten arc welding, as well as in the processing of and other reactive elements. An argon atmosphere is also used for growing crystals of and .

Argon is used in the poultry industry to birds, either for mass culling following disease outbreaks, or as a means of slaughter more humane than . Argon is denser than air and displaces oxygen close to the ground during inert gas asphyxiation. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen within the dead bird, argon also enhances shelf life.

Argon is sometimes used for extinguishing fires where valuable equipment may be damaged by water or foam.


Scientific research
Liquid argon is used as the target for neutrino experiments and direct searches. The interaction between the 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 the WIMP–nucleus scattering. As with most other liquefied noble gases, argon has a high scintillation light yield (about 51 photons/keV ), is transparent to its own scintillation light, and is relatively easy to purify. Compared to , argon is cheaper and has a distinct scintillation time profile, which allows the separation of electronic recoils from nuclear recoils. On the other hand, its intrinsic beta-ray background is larger due to contamination, unless one uses argon from underground sources, which has much less contamination. Most of the argon in Earth's atmosphere was produced by electron capture of long-lived ( + e → + ν) present in natural potassium within Earth. The activity in the atmosphere is maintained by cosmogenic production through the knockout reaction (n,2n) and similar reactions. The half-life of is only 269 years. As a result, the underground Ar, shielded by rock and water, has much less contamination. Dark-matter detectors currently operating with liquid argon include DarkSide, WArP, , microCLEAN and . Neutrino experiments include ICARUS and , both of which use high-purity liquid argon in a time projection chamber for fine grained three-dimensional imaging of neutrino interactions.

At Linköping University, Sweden, the inert gas is being utilized in a vacuum chamber in which plasma is introduced to ionize metallic films. This process results in a film usable for manufacturing computer processors. The new process would eliminate the need for chemical baths and use of expensive, dangerous and rare materials.


Preservative
Argon is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents (argon has the 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 , 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 (as with acetic acid bacteria) and standard chemistry.

Argon is sometimes used as the propellant in cans.

Argon is also used as a preservative for such products as , , and paint, by displacing air to prepare a container for storage.Zawalick, Steven Scott "Method for preserving an oxygen sensitive liquid product" Issue date: 7 October 2003.

Since 2002, the American National Archives stores 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.


Laboratory equipment
Argon may be used as the within and . Argon is preferred to less expensive nitrogen in cases where nitrogen may react with the reagents or apparatus.

Argon may be used as the carrier gas in gas chromatography and in electrospray ionization mass spectrometry; it is the gas of choice for the plasma used in ICP . Argon is preferred for the sputter coating of specimens for scanning electron microscopy. Argon gas is also commonly used for sputter deposition of thin films as in and for .


Medical use
procedures such as use liquid argon to destroy tissue such as cells. It is used in a procedure called "argon-enhanced coagulation", a form of argon . The procedure carries a risk of producing and has resulted in the death of at least one patient.

Blue are used in surgery to weld arteries, destroy tumors, and correct eye defects.

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.


Lighting
Incandescent lights are filled with argon, to preserve the filaments at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as in and calorimetry in experimental . Gas-discharge lamps filled with pure argon provide lilac/violet light; with argon and some mercury, blue light. Argon is also used for blue and green .


Miscellaneous uses
Argon is used for thermal insulation in energy-efficient windows. Argon is also used in technical to inflate a because it is inert and has low thermal conductivity.

Argon is used as a propellant in the development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). 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.

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily and dating. Also, potassium–argon dating and related argon-argon dating are used to date , , and .

Argon has been used by athletes as a doping agent to simulate hypoxic conditions. In 2014, the World Anti-Doping Agency (WADA) added argon and to the list of prohibited substances and methods, although at this time there is no reliable test for abuse.


Safety
Although argon is non-toxic, it is 38% more than air and therefore considered a dangerous in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless. A 1994 incident, in which a man was after entering an argon-filled section of oil pipe under construction in , highlights the dangers of argon tank leakage in confined spaces and emphasizes the need for proper use, storage and handling.


See also
  • Oxygen–argon ratio, a ratio of two physically similar gases, which has importance in various sectors.


Further reading


External links

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