Spectral lines of argon
|Name, symbol||argon, Ar|
|Appearance||colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field|
|Argon in the periodic table|
|Atomic number (Z)||18|
|Group, block||group 18 (noble gases), p-block|
|Element category||noble gas|
|Standard atomic weight (±) (Ar)||39.948(1)|
|Electron configuration||[Ne] 3s2 3p6|
|2, 8, 8|
|Melting point||83.81 K (−189.34 °C, −308.81 °F)|
|Boiling point||87.302 K (−185.848 °C, −302.526 °F)|
|Density at stp (0 °C and 101.325 kPa)||1.784 g/L|
|when liquid, at b.p.||1.3954 g/cm3|
|Triple point||83.8058 K, 68.89 kPa|
|Critical point||150.687 K, 4.863 MPa|
|Heat of fusion||1.18 kJ/mol|
|Heat of vaporization||6.53 kJ/mol|
|Molar heat capacity||20.85 J/(mol·K)|
|Electronegativity||Pauling scale: no data|
|Ionization energies||1st: 1520.6 kJ/mol
2nd: 2665.8 kJ/mol
3rd: 3931 kJ/mol
|Covalent radius||106±10 pm|
|Van der Waals radius||188 pm|
|Crystal structure||face-centered cubic (fcc)|
|Speed of sound||323 m/s (gas, at 27 °C)|
|Thermal conductivity||17.72×10−3 W/(m·K)|
|Discovery and first isolation||Lord Rayleigh and William Ramsay (1894)|
|Most stable isotopes of argon|
|Decay modes in parentheses are predicted, but have not yet been observed|
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 gas. Argon is the third most common gas in the Earth's atmosphere, at 0.934% (9,340 ppmv), making it over twice as abundant as the next most common atmospheric gas, water vapor (which averages about 4000 ppmv, but varies greatly), and 23 times as abundant as the next most common non-condensing atmospheric gas, carbon dioxide (400 ppmv), and more than 500 times as abundant as the next most common noble gas, neon (18 ppmv).
Nearly all of this argon 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, being the preferred argon isotope produced by stellar nucleosynthesis in supernovas. In addition, argon is the most prevalent of the noble gases in Earth's crust, with the element composing 0.00015% of this crust.
The name "argon" is derived from the Greek word αργον, neuter singular form of αργος 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 liquid air. Argon is mostly used as an inert shielding gas in welding and other 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. Argon gas also has uses in incandescent and fluorescent lighting, and other types of gas discharge tubes. Argon makes a distinctive blue-green gas laser. Argon is also used in fluorescent glow starters.
- 1 Characteristics
- 2 History
- 3 Occurrence
- 4 Isotopes
- 5 Compounds
- 6 Production
- 7 Applications
- 8 Safety
- 9 See also
- 10 References
- 11 Further reading
- 12 External links
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, and gas. Argon is chemically 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 fluorohydride (HArF), a compound of argon with fluorine and hydrogen which is stable below 17 K, was reported by researchers at the University of Helsinki in 2000. 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. Argon-containing ions and excited state complexes, such as ArH+
and ArF, respectively, are known to exist. Theoretical calculations have predicted several argon compounds that should be stable, but for which no synthesis routes are currently known.
Argon (’αργόν, neuter singular form of ’αργός, Greek meaning "inactive", in reference to its chemical inactivity) was suspected to be present in air by Henry Cavendish in 1785 but was not isolated until 1894 by Lord Rayleigh and Sir William Ramsay at University College London in an experiment in which they removed all of the oxygen, carbon dioxide, water and nitrogen from a sample of clean air. 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. 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".
Argon constitutes 0.934% by volume and 1.288% 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. The Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.
The main isotopes of argon found on Earth are 40
Ar (99.6%), 36
Ar (0.34%), and 38
Ar (0.06%). Naturally occurring 40
K, with a half-life of 1.25×109 years, decays to stable 40
Ar (11.2%) by electron capture or positron emission, and also to stable 40
Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks by the method of K-Ar dating.
In the Earth's atmosphere, 39
Ar is made by cosmic ray activity, primarily with 40
Ar. In the subsurface environment, it is also produced through neutron capture by 39
K or alpha emission by calcium. 37
Ar is created from the neutron spallation of 40
Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.
Argon is notable in that its isotopic composition varies greatly between different locations in the Solar System. Where the major source of argon is the decay of 40
K in rocks, 40
Ar will be the dominant isotope, as it is on Earth. Argon produced directly by stellar nucleosynthesis, in contrast, is dominated by the alpha process nuclide, 36
Ar. Correspondingly, solar argon contains 84.6% 36
Ar based on solar wind measurements, and the ratio of the three isotopes 36Ar : 38Ar : 40Ar in the atmospheres of the outer planets is measured to be 8400 : 1600 : 1. This contrasts with the abundance of primordial 36
Ar in Earth's atmosphere: only 31.5 ppmv (= 9340 ppmv × 0.337%), comparable to that of neon (18.18 ppmv); and with measurements by interplanetary probes.
The Martian atmosphere contains 1.6% of 40
Ar and 5 ppm of 36
Ar. The Mariner probe 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 discovered the presence of exclusively 40
Ar on Titan, the largest moon of Saturn.
The predominance of radiogenic 40
Ar is responsible for the standard atomic weight of terrestrial argon being greater than that of the next element, potassium, which was puzzling at the time when argon was discovered. Mendeleev had placed the elements in 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).
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 compound 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, argon fluorohydride (HArF) was formed. It is stable up to 40 kelvin (−233 °C). The metastable ArCF2+
2 dication, which is valence isoelectronic with carbonyl fluoride, was observed in 2010. Argon-36, in the form of argon hydride (argonium) ions, has been detected in cosmic dust 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 every year.
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.
There are several different reasons argon is used in particular applications:
- An inert gas is needed. In particular, argon is the cheapest alternative when 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 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 relatively cheap.
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 various 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.
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'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 within the dead bird, argon also enhances shelf life.
Argon is sometimes used for extinguishing fires where damage to equipment is to be avoided.
Liquid argon is used as the target for neutrino experiments and direct dark matter searches. The interaction of a hypothetical WIMP particle with the argon nucleus produces scintillation light that is detected by photomultiplier tubes. Two-phase detectors also use argon gas to detect the ionized electrons produced during the WIMP-nucleus scattering. As with most other liquefied noble gases, argon has a high scintillation lightyield (~ 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 39
Ar contamination, unless one uses underground argon sources which has much less 39
Ar contamination. Most of the argon in the Earth’s atmosphere was produced by electron capture of long-lived 40
K + e− → 40
Ar + ν) present in natural potassium within the earth. The 39
Ar activity in the atmosphere is maintained by cosmogenic production through 40
Ar and similar reactions. The half-life of 39
Ar is only 269 years. As a result, the underground Ar, shielded by rock and water, has much less 39
Ar contamination. Dark matter detectors currently operating with liquid argon include DarkSide, WArP, ArDM, microCLEAN and DEAP-I. 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.
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 of E938). Aerial oxidation, hydrolysis, and other chemical reactions which degrade the products are retarded or prevented entirely. Bottles of high-purity chemicals and certain pharmaceutical products are available in sealed bottles or ampoules packed in argon.
In winemaking, argon is used in a variety of activities to provide a barrier against oxygen at the liquid's surface, which can spoil wine by fueling both microbial metabolism (such as with acetic acid bacteria) and standard redox chemistry.
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. Using argon reduces gas leakage, compared with the helium used in the preceding five decades.
Argon may be used as the inert gas within Schlenk lines and gloveboxes. The use of argon over comparatively less expensive nitrogen is preferred where nitrogen may react with the experimental 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 liquefied argon to destroy tissue such as 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.
Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.
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, filled with argon and some mercury blue light. Argon is also used for the creation of blue and green laser light.
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 being 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 the AIM-9 Sidewinder missile, and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure.
Argon has been used by athletes as a doping agent to simulate hypoxic conditions. On August 31, 2014 the 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 is therefore considered a dangerous asphyxiant in closed areas. It is also 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.
- Standard Atomic Weights 2013. Commission on Isotopic Abundances and Atomic Weights
- Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 4.121. ISBN 1439855110.
- Shuen-Chen Hwang, Robert D. Lein, Daniel A. Morgan (2005). "Noble Gases". Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383. doi:10.1002/0471238961.0701190508230114.a01.
- Magnetic susceptibility of the elements and inorganic compounds, in Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
- In older versions of the periodic table, the noble gases were identified as Group VIIIA or as Group 0. See Group (periodic table).
- Material Safety Data Sheet Gaseous Argon, Universal Industrial Gases, Inc. Retrieved 14 October 2013.
- Leonid Khriachtchev; Mika Pettersson; Nino Runeberg; Jan Lundell; et al. (2000). "A stable argon compound". Nature 406: 874–876. doi:10.1038/35022551. PMID 10972285.
- Perkins, S. (26 August 2000). "HArF! Argon's not so noble after all – researchers make argon fluorohydride". Science News.
- Belosludov, V. R.; Subbotin, O. S.; Krupskii, D. S.; Prokuda, O. V.; et al. (2006). "Microscopic model of clathrate compounds". Journal of Physics: Conference Series 29: 1. Bibcode:2006JPhCS..29....1B. doi:10.1088/1742-6596/29/1/001.
- Cohen, A.; Lundell, J.; Gerber, R. B. (2003). "First compounds with argon–carbon and argon–silicon chemical bonds". Journal of Chemical Physics 119 (13): 6415. Bibcode:2003JChPh.119.6415C. doi:10.1063/1.1613631.
- Hiebert, E. N. (1963). "In Noble-Gas Compounds". In Hyman, H. H. Historical Remarks on the Discovery of Argon: The First Noble Gas. University of Chicago Press. pp. 3–20.
- Travers, M. W. (1928). The Discovery of the Rare Gases. Edward Arnold & Co. pp. 1–7.
- Lord Rayleigh; Ramsay, William (1894–1895). "Argon, a New Constituent of the Atmosphere". Proceedings of the Royal Society 57 (1): 265–287. doi:10.1098/rspl.1894.0149. JSTOR 115394.
- Lord Rayleigh; Ramsay, William (1895). "VI. Argon: A New Constituent of the Atmosphere". Philosophical Transactions of the Royal Society A 186: 187. Bibcode:1895RSPTA.186..187R. doi:10.1098/rsta.1895.0006. JSTOR 90645.
- Ramsay, W. (1904). "Nobel Lecture". The Nobel Foundation.
- "About Argon, the Inert; The New Element Supposedly Found in the Atmosphere". The New York Times. 3 March 1895. Retrieved 1 February 2009.
- Holden, N. E. (12 March 2004). "History of the Origin of the Chemical Elements and Their Discoverers". National Nuclear Data Center.
- "Encyclopædia Britannica Online, s.v. "argon (Ar)"". Retrieved 14 January 2014.
- "Argon, Ar". Etacude.com. Retrieved 8 March 2007.
- Emsley, J. (2001). Nature's Building Blocks. Oxford University Press. pp. 44–45. ISBN 978-0-19-960563-7.
- "40Ar/39Ar dating and errors". Archived from the original on 9 May 2007. Retrieved 7 March 2007.
- Lodders, K. (2008). "The solar argon abundance". Astrophysical Journal 674: 607. arXiv:0710.4523. Bibcode:2008ApJ...674..607L. doi:10.1086/524725.
- Cameron, A. G. W. (1973). "Elemental and isotopic abundances of the volatile elements in the outer planets". Space Science Reviews 14 (3–4): 392–400. Bibcode:1973SSRv...14..392C. doi:10.1007/BF00214750.
- "Seeing, touching and smelling the extraordinarily Earth-like world of Titan". European Space Agency. 21 January 2005.
- Kean, Sam (2011). "Chemistry Way, Way Below Zero". The Disappearing Spoon. Black Bay Books.
- Bartlett, Neil (8 September 2003). "The Noble Gases". Chemical & Engineering News 81 (36).
- Lockyear, JF; Douglas, K; Price, SD; Karwowska, M; et al. (2010). "Generation of the ArCF22+ Dication". Journal of Physical Chemistry Letters 1: 358. doi:10.1021/jz900274p.
- Barlow, M. J.; et al. (2013). "Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula". Science 342 (6164): 1343–1345. arXiv:1312.4843. Bibcode:2013Sci...342.1343B. doi:10.1126/science.1243582.
- Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". New York Times. Retrieved 13 December 2013.
- Kleppe, Annette K.; Amboage, Mónica; Jephcoat, Andrew P. (2014). "New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system". Scientific Reports 4. doi:10.1038/srep04989.
- "Periodic Table of Elements: Argon – Ar". Environmentalchemistry.com. Retrieved 12 September 2008.
- Fletcher, D. L. "Slaughter Technology" (PDF). Symposium: Recent Advances in Poultry Slaughter Technology. Retrieved 1 January 2010.
- Gastler, Dan; Kearns, Ed; Hime, Andrew; Stonehill, Laura C.; et al. (2012). "Measurement of scintillation efficiency for nuclear recoils in liquid argon". Physical Review C 85 (6). arXiv:1004.0373. Bibcode:2012PhRvC..85f5811G. doi:10.1103/PhysRevC.85.065811.
- Xu, J.; Calaprice, F.; Galbiati, C.; Goretti, A.; Guray, G.; et al. (2012). "A Study of the Residual 39Ar Content in Argon from Underground Sources". arXiv:1204.6011v1.
- Zawalick, Steven Scott "Method for preserving an oxygen sensitive liquid product" U.S. Patent 6,629,402 Issue date: 7 October 2003
- "Schedule for Renovation of the National Archives Building". Retrieved 7 July 2009.
- "Fatal Gas Embolism Caused by Overpressurization during Laparoscopic Use of Argon Enhanced Coagulation". MDSR. 24 June 1994.
- Pilmanis Andrew A; Balldin UI; Webb James T; Krause KM (2003). "Staged decompression to 3.5 psi using argon-oxygen and 100% oxygen breathing mixtures". Aviation, Space, and Environmental Medicine 74 (12): 1243–50. PMID 14692466.
- "Energy-Efficient Windows". FineHomebuilding.com. Retrieved 1 August 2009.
- Nuckols ML; Giblo J; Wood-Putnam JL (15–18 September 2008). "Thermal Characteristics of Diving Garments When Using Argon as a Suit Inflation Gas". Proceedings of the Oceans 08 MTS/IEEE Quebec, Canada Meeting (MTS/IEEE). Retrieved 2 March 2009.
- "Description of Aim-9 Operation". planken.org. Archived from the original on 22 December 2008. Retrieved 1 February 2009.
- "WADA amends Section S.2.1 of 2014 Prohibited List". 31 August 2014.
- Alaska FACE Investigation 94AK012 (23 June 1994). "Welder's Helper Asphyxiated in Argon-Inerted Pipe – Alaska (FACE AK-94-012)". State of Alaska Department of Public Health. Retrieved 29 January 2011.
- Brown, T. L.; Bursten, B. E.; LeMay, H. E. (2006). J. Challice; N. Folchetti, eds. Chemistry: The Central Science (10th ed.). Pearson Education. pp. 276 & 289. ISBN 978-0-13-109686-8.
- Triple point temperature: 83.8058 K – Preston-Thomas, H. (1990). "The International Temperature Scale of 1990 (ITS-90)". Metrologia 27: 3–10. Bibcode:1990Metro..27....3P. doi:10.1088/0026-1394/27/1/002.
- Triple point pressure: 69 kPa – Lide, D. R. (2005). "Properties of the Elements and Inorganic Compounds; Melting, boiling, triple, and critical temperatures of the elements". CRC Handbook of Chemistry and Physics (86th ed.). CRC Press. §4. ISBN 0-8493-0486-5.
- Argon at The Periodic Table of Videos (University of Nottingham)
- USGS Periodic Table – Argon
- Diving applications: Why Argon?
|Periodic table (Large cells)|