This article pertains to the chemical element. For other uses, see argon (disambiguation).

18 chlorine ← argon → potassium Ne ↑ Ar ↓ Kr Periodic table - Extended periodic table
General
Name, symbol, number	argon, Ar, 18
Chemical series	noble gases
Group, period, block	18, 3, p
Appearance	colorless
Standard atomic weight	39.948(1) gmol−1
Electron configuration	[Ne] 3s2 3p6
Electrons per shell	2, 8, 8
Physical properties
Phase	gas (At room temperature)
Density	(0 C, 101.325 kPa) 1.784 g/L
Melting point	83.80 K (−189.35 °C, −308.83 °F)
Boiling point	87.30 K (−185.85 °C, −302.53 °F)
Triple point	83.8058 K, 69 kPa
Critical point	150.87 K, 4.898 MPa
Heat of fusion	1.18 kJmol−1
Heat of vaporization	6.43 kJmol−1
Heat capacity	(25 C) 20.786 Jmol−1K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K   47 53 61 71 87
Atomic properties
Crystal structure	cubic face centered
Oxidation states	0
Electronegativity	no data (Pauling scale)
Ionization energies (more)	1st: 1520.6 kJmol−1
	2nd: 2665.8 kJmol−1
	3rd: 3931 kJmol−1
Atomic radius	71 pm
Atomic radius (calc.)	71 pm
Covalent radius	97 pm
Van der Waals radius	188 pm
Miscellaneous
Magnetic ordering	nonmagnetic
Thermal conductivity	(300 K) 17.72 m Wm−1K−1
Speed of sound	(gas, 27 C) 323 m/s
CAS registry number	7440–37–1
Selected isotopes
Main article: Isotopes of argon iso NA half-life DM DE (MeV) DP 36Ar 0.337% Ar is stable with 18 neutrons 37Ar syn 35 d ε 0.813 37Cl 38Ar 0.063% Ar is stable with 20 neutrons 39Ar syn 269 y β- 0.565 39K 40Ar 99.600% Ar is stable with 22 neutrons 41Ar syn 109.34 min β- 2.49 41K 42Ar syn 32.9 y β- 0.600 42K
References
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Argon (IPA:/ˈɑːgɒn/) is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). Argon is present in the Earth's atmosphere at slightly less than 1%, making it the most common noble gas on Earth. Its full outer 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.

Characteristics

Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas. This highly stable chemical element is colorless, odorless, tasteless and nontoxic in both its liquid and gaseous forms. Argon is inert under most conditions and forms no confirmed stable compounds at room temperature.

Although Argon is a noble gas, it has been found to have the capability of forming some compounds. For example, the creation of argon hydrofluoride (HArF), a metastable compound of argon with fluorine and hydrogen, has been reported by researchers at the University of Helsinki in 2000.^[1] Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules.^[2] Also argon-containing ions e.g. ArH+ and excited state complexes e.g. ArF are well known. Theoretical calculations on computers have shown several argon compounds that should be stable but for which no synthesis routes are currently known.

History

Argon (Greek αργόν meaning "the lazy one," in reference to its chemical inactivity)^[3][4][5] was suspected to be present in air by Henry Cavendish in 1785 but was not discovered until 1894 by Lord Rayleigh and Sir William Ramsay in an experiment in which they removed all of the oxygen and nitrogen from a sample of air.^[6] Argon was also encountered in 1882 through independent research of H.F. Newall and W.N. Hartley. Each observed new lines in the color spectrum of air but were unable to identify the element responsible for the lines. Argon became the first member of the noble gases to be discovered. The symbol for Argon is now Ar, but up until 1957 it was A.^[7]

Applications

There are several different reasons why argon is used in particular applications:

• A very inert gas is required, particularly where diatomic nitrogen is not sufficiently inert.
• Low thermal conductivity is required.
• The electronic properties (ionization and/or the emission spectrum) are needed.

Other noble gases would probably work as well in most of these applications, but argon is by far the cheapest. Argon is inexpensive since it is a byproduct of the production of liquid oxygen and liquid nitrogen, both of which 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 since it has the highest concentration in the atmosphere.

The bulk of argon applications arise simply because it is inert and relatively cheap. Argon is used

• As a fill gas in incandescent lighting, since argon will not react with the filament of light bulbs even at high temperatures.
• As an inert gas shield in many forms of welding, including metal inert gas welding and tungsten inert gas welding.
• For extinguishing fires where damage to equipment is to be avoided (see photo).
• As the gas of choice for the plasma used in ICP spectroscopy
• As a non-reactive blanket in the processing of titanium and other reactive elements,
• As a protective atmosphere for growing silicon and germanium crystals, and in partial pressure heat treat furnaces.
• By museum conservators to protect old materials or documents, which are prone to gradual oxidation in the presence of air. ^[8]
• To keep open bottles of wine from oxidizing, and in a number of dispensing units and keeper cap systems.
• In winemaking to top off barrels, displacing oxygen and thus preventing the wine from turning to vinegar during the aging process.
• Used to cool the seeker head of the US Air Force version of the AIM-9 Sidewinder missile. The gas is stored at high pressure, and the expansion of the gas cools the seeker^[9].

The next most common reason for using argon is its low thermal conductivity. It is used for thermal insulation in energy efficient windows.^[10] Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.

Argon is also used for the specific way it ionizes and emits light. It is used in plasma globes and calorimetry in experimental particle physics. Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.^[11] In microelectronics, argon ions are used for sputtering.

Finally, there are a number of miscellaneous uses. Argon-39, with a half life of 269 years, has been used for a number of applications, primarily ice core and ground water dating. The Argon-40/Potassium-40 ratio is used in dating igneous rocks.

Cryosurgery procedures such as cryoablation use liquified argon to destroy cancer cells. In surgery it is used in a procedure called "argon enhanced coagulation" which is a form of argon plasma beam electrosurgery. The procedure carries a risk of producing gas embolism in the patient and has resulted in the death of one person via this type of accident. ^[12]

Occurrence

Argon constitutes 0.934% by volume and 1.29% by mass of the Earth's atmosphere, and air is the primary raw material used by industry to produce purified argon products. 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.^[13]

The Martian atmosphere in contrast contains 1.6% of argon-40 and 5 ppm of argon-36. The Mariner spaceprobe fly-by of the planet Mercury in 1973 found that Mercury has a very thin atmosphere with 70% argon, believed to result from releases of the gas as a decay product from radioactive materials on the planet. In 2005, the Huygens probe also discovered the presence of argon-40 on Titan, the largest moon of Saturn.^[14]

Compounds

Argon’s complete octet of electrons indicates full s and p subshells. This full outer energy level 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. In August 2000, the first argon compounds were formed by researchers at the University of Helsinki. By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, argon hydrofluoride (HArF) was formed.^[15] It is stable up to 40 kelvins (−233 Â°C). The discovery of argon difluoride (ArF₂) was announced in 2003.

Isotopes

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 1.2510⁹ years, decays to stable ⁴⁰Ar (11.2%) by electron capture and positron emission, and also to stable ⁴⁰Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks.^[16]

In the Earth's atmosphere, ³⁹Ar is made by cosmic ray activity, primarily with ⁴⁰Ar. In the subsurface environment, it is also produced through neutron capture by ³⁹K or alpha emission by calcium. ³⁷Ar is created from the decay of ⁴⁰Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.^[16]

Potential hazards

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is a simple asphyxiant. People have suffocated by breathing argon by mistake.^[17]

References

Further reading

• Los Alamos National Laboratory – Argon
• USGS Periodic Table - Argon
• Emsley, J., Nature’s Building Blocks; Oxford University Press: Oxford, NY, 2001; pp. 35-39.
• Brown, T. L.; Bursten, B. E.; LeMay, H. E., In Chemistry: The Central Science, 10th ed.; Challice, J.; Draper, P.; Folchetti, N. et al.; Eds.; Pearson Education, Inc.: Upper Saddle River, NJ, 2006; pp. 276 and 289.
• Triple point temperature: 83.8058 K - Preston-Thomas, H. (1990). "The International Temperature Scale of 1990 (ITS-90)". Metrologia 27: 3-10. 
• Triple point pressure: 69 kPa - (2005) "Section 4, Properties of the Elements and Inorganic Compounds; Melting, boiling, triple, and critical temperatures of the elements", CRC Handbook of Chemistry and Physics, 85th edition, Boca Raton, Florida: CRC Press. 

External links

• WebElements.com – Argon
• Diving applications: Why Argon?
• Argon Ar Properties, Uses, Applications

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