Argon

18 chlorine ← argon → potassium Ne ↑ Ar ↓ Kr Periodic table - Extended periodic table
General
Name, symbol, number	argon, Ar, 18
Element category	noble gases
Group, period, block	18, 3, p
Appearance	colourless gas
Standard atomic weight	39.950(1) g·mol−1
Electron configuration	[Ne] 3s2 3p6
Electrons per shell	2, 8, 8
Physical properties
Phase	gas
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 (-189°C), 69 kPa
Critical point	150.87 K, 4.898 MPa
Heat of fusion	1.18 kJ·mol−1
Heat of vaporization	6.43 kJ·mol−1
Specific heat capacity	(25 °C) 20.786 J·mol−1·K−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 kJ·mol−1
	2nd: 2665.8 kJ·mol−1
	3rd: 3931 kJ·mol−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) 17.72x10-3  W·m−1·K−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% 36Ar is stable with 18 neutrons 37Ar syn 35 d ε 0.813 37Cl 38Ar 0.063% 38Ar is stable with 20 neutrons 39Ar syn 269 y β- 0.565 39K 40Ar 99.600% 40Ar is stable with 22 neutrons 41Ar syn 109.34 min β- 2.49 41K 42Ar syn 32.9 y β- 0.600 42K
References

Argon (pronounced /ˈɑrgɒn/) 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 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases. Argon's full outer shell makes it 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. Argon 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, was 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 and excited state complexes, such as ArH+ and ArF, respectively, are known to exist. Theoretical calculations have shown several argon compounds that should be stable but for which no synthesis routes are currently known.

History

Cavendish's method for the isolation of Argon. 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 rouind 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 meaning "inactive," 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 Scotland in an experiment in which they removed all of the oxygen and nitrogen from a sample of air.^[6][7] They had determined that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, 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.^[8] 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.^[9]

Occurrence

An argon & mercury vapour discharge tube.

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.^[10]

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.^[11]

Isotopes

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 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.^[12]

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.^[12]

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.^[13] It is stable up to 40 kelvins (−233 °C).

See also:H₂-Ar

A small piece of rapidly melting argon ice.

Production

Industrial

Argon is produced industrially by the partial distillation of liquid air, a process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K and oxygen, which boils at 90.2 K. About 700,000 tons of argon are produced worldwide every year. ^[14]

In radioactive decays

Argon-40, the most abundant isotope of argon, is produced by the decay of potassium-40 with a half-life of 1.26×10⁹ years by electron capture or positron emission. Because of this, it is used in potassium-argon dating to determine the age of rocks.

Applications

Cylinders containing argon gas for use in extinguishing fire without damaging server equipment

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

• An inert gas is needed. In particular, argon is the cheapest alternative when diatomic nitrogen is not sufficiently inert.
• Low thermal conductivity is required.
• The electronic properties (ionization and/or the emission spectrum) are necessary.

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, because 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 metal inert gas welding argon is often mixed with CO₂.
• 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 the gas of choice (in ionised form) for sputter coating of specimens for Scanning Electron Microscopy.
• 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.^[15]
• 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.
• In the pharmaceutical industry to top off bottles of intravenous drug preparations (for example intravenous paracetamol), again displacing oxygen and therefore prolonging the drug's shelf-life.
• Used to cool the seeker head of the US Air Force version of the AIM-9 Sidewinder missile, and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure, and the expansion of the gas cools the seeker.^[16]
• As an atmosphere in graphite electric furnaces, to keep graphite from oxidizing.
• As an asphyxiant in the poultry industry, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Argon's relatively high density causes it to remain close to the ground during gassing. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen at a cellular level in the dead bird, argon also enhances shelf life.^[17]
• As filling of vapour discharge tubes with blue colour .

It is used for thermal insulation in energy efficient windows.^[18] Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.^[19] It has also used experimentally to replace nitrogen in the breathing or decompression mix, to speed the elimination of dissolved nitrogen from the blood.^[20] See Argox (scuba).

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.^[21] 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. Also, potassium-argon dating 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.^[22]

Potential hazards

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is a simple asphyxiant, and, in confined spaces, is known to result in death due to asphyxiation. A recent fatality in Alaska highlights the dangers of argon tank leakage in confined spaces, and, emphasizes the need for its proper use, storage and handling. Argon is also odorless, tastless, and colorless,making it hard to detect.^[23] Argon is 25% heavier than air and thus it's considered most dangerous in closed areas.

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. doi:10.1088/0026-1394/27/1/002. http://www.bipm.org/en/publications/its-90.html. 
• Triple point pressure: 69 kPa - "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 ed.). Boca Raton, Florida: CRC Press. 2005. 

External links

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