Argon is a chemical element with symbol Ar and atomic
number 18. It is in group 18 of the periodic table and is a noble
Argon is the third-most abundant gas in the Earth's
atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant
as water vapor (which averages about 4000 ppmv, but varies greatly),
23 times as abundant as carbon dioxide (400 ppmv), and more than 500
times as abundant as neon (18 ppmv).
Argon is the most abundant noble
gas in Earth's crust, comprising 0.00015% of the crust.
Nearly all of the argon in the
Earth's atmosphere is radiogenic
argon-40, derived from the decay of potassium-40 in the Earth's crust.
In the universe, argon-36 is by far the most common argon isotope, as
it is the most easily produced by stellar nucleosynthesis in
The name "argon" is derived from the Greek 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 octet (eight electrons) in the outer atomic
shell makes argon stable and resistant to bonding with other elements.
Its triple point temperature of 83.8058 K is a defining fixed
point in the International Temperature Scale of 1990.
Argon is produced industrially by the fractional distillation of
Argon is mostly 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.
Argon is also used in incandescent, fluorescent
lighting, and other gas-discharge tubes.
Argon makes a distinctive
blue-green gas laser.
Argon is also used in fluorescent glow starters.
6.2 In radioactive decays
7.1 Industrial processes
7.2 Scientific research
7.4 Laboratory equipment
7.5 Medical use
7.7 Miscellaneous uses
9 See also
11 Further reading
12 External links
A small piece of rapidly melting solid argon
Argon has approximately the same solubility in water as oxygen and is
2.5 times more soluble in water than nitrogen.
Argon is colorless,
odorless, nonflammable and nontoxic as a solid, liquid or gas.
Argon is chemically inert under most conditions and forms no confirmed
stable compounds at room temperature.
Although argon is a noble gas, it can form some compounds under
Argon fluorohydride (HArF), a compound of argon
with fluorine and hydrogen that is stable below 17 K
(−256.1 °C; −429.1 °F), has been demonstrated.
Although the neutral ground-state chemical compounds of argon are
presently limited to HArF, argon can form clathrates with water when
atoms of argon are trapped in a lattice of water molecules. Ions,
such as ArH+, and excited-state complexes, such as ArF, have been
demonstrated. Theoretical calculation predicts several more argon
compounds that should be stable but have not yet been synthesized.
Lord Rayleigh's method for the isolation of argon, based on an
experiment of Henry Cavendish's. The gases are contained in a
test-tube (A) standing over a large quantity of weak alkali (B), and
the current is conveyed in wires insulated by U-shaped glass tubes
(CC) passing through the liquid and round the mouth of the test-tube.
The inner platinum ends (DD) of the wire receive a current from a
battery of five Grove cells and a
Ruhmkorff coil of medium size.
Argon (Greek ἀργόν, neuter singular form of ἀργός meaning
"lazy" or "inactive"), is named in reference to its chemical
inactivity. This chemical property of this first noble gas to be
discovered impressed the namers. An unreactive gas was
suspected to be a component of air by
Henry Cavendish in 1785. Argon
was first isolated from air in 1894 by
Lord Rayleigh and Sir William
University College London
University College London by removing oxygen, carbon
dioxide, water, and nitrogen from a sample of clean air.
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. Each observed new lines in the emission
spectrum of air that did not match known elements.
Until 1957, the symbol for argon was "A", but now is "Ar".
Argon constitutes 0.934% by volume and 1.288% by mass of the Earth's
atmosphere, and air is the primary industrial source of purified
Argon is isolated from air by fractionation, most
commonly by cryogenic fractional distillation, a process that also
produces purified nitrogen, oxygen, neon, krypton and xenon. The
Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon,
Main article: Isotopes of argon
The main isotopes of argon found on Earth are 40Ar (99.6%), 36Ar
(0.34%), and 38Ar (0.06%). Naturally occurring 40K, with a half-life
of 1.25×109 years, decays to stable 40Ar (11.2%) by electron capture
or positron emission, and also to stable 40Ca (88.8%) by beta decay.
These properties and ratios are used to determine the age of rocks by
In the Earth's atmosphere, 39Ar is made by cosmic ray activity,
primarily by neutron capture of 40Ar followed by two-neutron emission.
In the subsurface environment, it is also produced through neutron
capture by 39K, followed by proton emission. 37Ar is created from the
neutron capture by 40Ca followed by an alpha particle emission as a
result of subsurface nuclear explosions. It has a half-life of 35
Between locations in the Solar System, the isotopic composition of
argon varies greatly. Where the major source of argon is the decay of
40K in rocks, 40Ar will be the dominant isotope, as it is on Earth.
Argon produced directly by stellar nucleosynthesis, is dominated by
the alpha-process nuclide 36Ar. Correspondingly, solar argon contains
84.6% 36Ar (according to solar wind 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 36Ar 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
Saturn) contain argon, predominantly as 40Ar, and its content may be
as high as 1.93% (Mars).
The predominance of radiogenic 40Ar is the reason the standard atomic
weight of terrestrial argon is greater than that of the next element,
potassium, a fact that was puzzling when argon was discovered.
Mendeleev positioned the elements on his periodic table in order of
atomic weight, but the inertness of argon suggested a placement before
the reactive alkali metal.
Henry Moseley later solved this problem by
showing that the periodic table is actually arranged in order of
atomic number (see History of the periodic table).
Space-filling model of argon fluorohydride
Argon's complete octet of electrons indicates full s and p subshells.
This full valence shell 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 caesium iodide.
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 metastable ArCF2+
2 dication, which is valence-isoelectronic with carbonyl fluoride and
phosgene, was observed in 2010. Argon-36, in the form of argon
hydride (argonium) ions, has been detected in interstellar medium
associated with the
Crab Nebula supernova; this was the first
noble-gas molecule detected in outer space.
Solid argon hydride (Ar(H2)2) has the same crystal structure as the
MgZn2 Laves phase. 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.
Argon is produced industrially by the fractional distillation of
liquid air in a cryogenic air separation unit; a process that
separates liquid nitrogen, which boils at 77.3 K, from argon,
which boils at 87.3 K, and liquid oxygen, which boils at
90.2 K. About 700,000 tonnes of argon are produced worldwide
In radioactive decays
40Ar, the most abundant isotope of argon, is produced by the decay of
40K with a half-life of 1.25×109 years by electron capture or
positron emission. Because of this, it is used in potassium–argon
dating to determine the age of rocks.
Cylinders containing argon gas for use in extinguishing fire without
damaging server equipment
Argon has several desirable properties:
Argon is a chemically inert gas.
Argon is the cheapest alternative when nitrogen is not sufficiently
Argon has low thermal conductivity.
Argon has electronic properties (ionization and/or the emission
spectrum) desirable for some applications.
Other noble gases would be equally suitable for most of these
applications, but argon is by far the cheapest.
Argon is inexpensive,
since it occurs naturally in air and is readily obtained as a
byproduct of cryogenic air separation in the production of liquid
oxygen and liquid nitrogen: the primary constituents of air are used
on a large industrial scale. The other noble gases (except helium) are
produced this way as well, but argon is the most plentiful by far. The
bulk of argon applications arise simply because it is inert and
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 arc welding such as gas metal arc welding and gas tungsten arc
welding, as well as in the processing of titanium and other reactive
elements. An argon atmosphere is also used for growing crystals of
silicon and germanium.
See also: shielding gas
Argon is used in the poultry industry to asphyxiate birds, either for
mass culling following disease outbreaks, or as a means of slaughter
more humane than the electric bath.
Argon is denser than air and
displaces oxygen close to the ground during gassing. Its
non-reactive nature makes it suitable in a food product, and since it
replaces oxygen within the dead bird, argon also enhances shelf
Argon is sometimes used for extinguishing fires where valuable
equipment may be damaged by water or foam.
Liquid argon is used as the target for neutrino experiments and direct
dark matter 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 xenon, 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 39Ar contamination, unless one
uses argon from underground sources, which has much less 39Ar
contamination. Most of the argon in the Earth’s atmosphere was
produced by electron capture of long-lived 40K (40K + e− → 40Ar +
ν) present in natural potassium within the Earth. The 39Ar activity
in the atmosphere is maintained by cosmogenic production through the
knockout reaction 40Ar(n,2n)39Ar and similar reactions. The half-life
of 39Ar is only 269 years. As a result, the underground Ar,
shielded by rock and water, has much less 39Ar contamination.
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.
A sample of caesium is packed under argon to avoid reactions with air
Argon is used to displace oxygen- and moisture-containing air in
packaging material to extend the shelf-lives of the contents (argon
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 (as with acetic acid bacteria) and
standard redox chemistry.
Argon is sometimes used as the propellant in aerosol cans for such
products as varnish, polyurethane, and paint, and to displace air when
preparing a container for storage after opening.
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.
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.
Gloveboxes are often filled with argon, which recirculates over
scrubbers to maintain an oxygen-, nitrogen-, and moisture-free
See also: Air-free technique
Argon may be used as the inert gas within Schlenk lines 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 spectroscopy.
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
microelectronics and for wafer cleaning in microfabrication.
Cryosurgery procedures such as cryoablation use liquid argon to
destroy tissue such as cancer cells. 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.
Blue argon lasers 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.
Argon gas-discharge lamp forming the symbol for argon "Ar"
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 plasma globes and calorimetry in
experimental particle physics. Gas-discharge lamps filled with pure
argon provide lilac/violet light; with argon and some mercury, blue
Argon is also used for blue and green argon-ion lasers.
Argon is used for thermal insulation in energy-efficient windows.
Argon is also used in technical scuba diving to inflate a dry suit
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 ice core and ground water dating. Also,
potassium–argon dating and related argon-argon dating is used to
date sedimentary, metamorphic, and igneous rocks.
Argon has been used by athletes as a doping agent to simulate hypoxic
conditions. In 2014, the
World Anti-Doping Agency
World Anti-Doping Agency (WADA) added argon
and xenon to the list of prohibited substances and methods, although
at this time there is no reliable test for abuse.
Although argon is non-toxic, it is 38% denser than air and therefore
considered a dangerous asphyxiant in closed areas. It is difficult to
detect because it is colorless, odorless, and tasteless. 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.
Oxygen–argon ratio, a ratio of two physically similar gases, which
has importance in various sectors.
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