Argon

This article is about the chemical element. For other uses, see Argon (disambiguation). chlorine ← argon → potassium Ne ↑ Ar ↓ Kr 18Ar Periodic table Appearance colorless gas exhibiting an lilac/violet glow when placed in a high voltage electric field Spectral lines of argon. General properties Name, symbol, number argon, Ar, 18 Pronunciation /ˈɑrɡɒn/ Element category noble gases Group, period, block 18, 3, p Standard atomic weight 39.948g·mol−1 Electron configuration Ne 3s2 3p6 Electrons per shell 2, 8, 8 (Image) Physical properties Phase gas Density (0 °C, 101.325 kPa) 1.784 g/L Liquid density at b.p. 1.40 g·cm−3 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 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 Covalent radius 106±10 pm Van der Waals radius 188 pm Miscellanea Crystal structure face-centered cubic Magnetic ordering diamagnetic1 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 Most stable 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 trace 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 v · d · e Argon ( /ˈɑrɡɒn/) is a chemical element represented 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 the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide. 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. The name "argon" is derived from the Greek word αργον meaning "the inactive one", 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. Contents 1 Characteristics 2 History 3 Occurrence 4 Isotopes 5 Compounds 6 Production 6.1 Industrial 6.2 In radioactive decays 7 Applications 7.1 Industrial processes 7.2 Scientific research 7.3 Preservative 7.4 Laboratory equipment 7.5 Medical use 7.6 Lighting 7.7 Miscellaneous uses 8 Safety 9 References 10 Further reading 11 External links // Characteristics A small piece of rapidly melting argon ice. 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, and nontoxic as a solid, liquid, and gas. 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 fluorohydride (HArF), a marginally stable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000.2 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.3 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,4 but for which no synthesis routes are currently known. History 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 (αργος, Greek meaning "inactive", in reference to its chemical inactivity)56 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 in Scotland in an experiment in which they removed all of the oxygen, carbon dioxide, water and nitrogen from a sample of clean air.789 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.10 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.11 Occurrence Argon constitutes 0.934% by volume and 1.28% 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.12 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 by the method of K-Ar dating.13 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 neutron spallation of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.13 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 potassium-40 in rocks, Argon-40 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, argon-36. Correspondingly, solar argon contains 84.6% argon-36 based on solar wind measurements.14 The predominance of radiogenic argon-40 is responsible for the fact that the standard atomic weight of terrestrial argon is greater than that of the next element, potassium. This was puzzling at the time when argon was discovered, since Mendeleev had placed the elements in his periodic table in order of atomic weight, although the inertness of argon implies that it must be placed before the reactive alkali metal potassium. 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). The much greater atmospheric abundance of argon relative to the other noble gases is also due to the presence of radiogenic argon-40. Primordial argon-36 has an abundance of only 31.5 ppmv (= 9340 ppmv x 0.337%), comparable to that of neon (18.18 ppmv). The Martian atmosphere contains 1.6% of argon-40 and 5 ppm of argon-36. The Mariner space 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 also discovered the presence of argon-40 on Titan, the largest moon of Saturn.15 Compounds See also: H2-Ar 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 fluorohydride (HArF) was formed.216 It is stable up to 40 kelvin (−233 °C). The ArCF2+ 2 metastable dication was also observed.17 Production Industrial 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.18 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. Applications Cylinders containing argon gas for use in extinguishing fire without damaging server equipment 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 is a byproduct of the production of liquid oxygen and liquid nitrogen from a cryogenic air separation unit, 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. 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 various types of metal inert gas welding such as tungsten inert gas 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 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 within the dead bird, argon also enhances shelf life.19 Argon is sometimes used for extinguishing fires where damage to equipment is to be avoided. Scientific research Argon is used, primarily in liquid form, as the target for direct dark matter searches. The interaction of a hypothetical WIMP particle with the Argon nucleus produces scintillation light that is then 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 20), 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 gamma-ray background is larger due to 39Ar contamination, unless one uses underground Argon sources with a low level of radioactivity. Dark matter detectors currently operating with liquid Argon include WArP, ArDM, microCLEAN and DEAP-I. Preservative 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. 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 wine making, argon is used to top-off barrels to avoid the aerial oxidation of ethanol to acetic acid during the aging process. Argon is also available in aerosol-type cans, which may be used to preserve compounds such as varnish, polyurethane, paint, etc. for storage after opening.21 Since 2001, the American National Archives stores important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to retard their degradation. Using argon reduces gas leakage, compared with the helium used in the preceding five decades.22 Laboratory equipment Gloveboxes are often filled with argon, which recirculates over scrubbers to maintain an oxygen- and moisture-free atmosphere See also: Air-free technique 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 ions are also used for sputtering in microelectronics. Medical use Cryosurgery procedures such as cryoablation use liquefied 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.23 Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.24 It has also used experimentally to replace nitrogen in the breathing or decompression mix, to speed the elimination of dissolved nitrogen from the blood.25 See Argox (breathing gas). Lighting Argon discharge tube Argon gas-discharge lamp forming the symbol for Argon "Ar". Small amounts of mercury are sometimes added to argon to produce an electric blue color. 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 argon provide blue light. Argon is also used for the creation of blue laser light. Miscellaneous uses It is used for thermal insulation in energy efficient windows.26 Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.27 Compressed argon 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.28 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. Safety Although argon is non-toxic, it does not satisfy the body's need for oxygen and is thus an asphyxiant. Argon is 25% more dense than air and is considered highly dangerous in closed areas. It is also difficult to detect because it is colorless, odorless, and tasteless. In confined spaces, it is known to result in death due to asphyxiation. 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.29 References ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press. ^ a b "HArF! Argon's not so noble after all – researchers make argon fluorohydride". Science News. 2000. http://findarticles.com/p/articles/mi_m1200/is_9_158/ai_65368548.  ^ Belosludov, V. R.; Subbotin, O S; Krupskii, D S; Prokuda, O V; Belosludov, R V; Kawazoe, Y (2006). "Microscopic model of clathrate compounds". J. Phys.: Conf. Ser. 29: 1. doi:10.1088/1742-6596/29/1/001.  ^ Cohen, Arik; Lundell, Jan; Gerber, R. Benny (2003). "First compounds with argon–carbon and argon–silicon chemical bonds". The Journal of Chemical Physics 119: 6415. 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. Chicago, IL: University of Chicago Press. pp. 3–20.  ^ Travers, M. W. (1928). The Discovery of the Rare Gases. London: Edward Arnold & Co.. pp. 1–7.  ^ Lord Rayleigh; Ramsay, William (1894–1895). "Argon, a New Constituent of the Atmosphere". Proceedings of the Royal Society of London 57 (1): 265–287. doi:10.1098/rspl.1894.0149. http://www.jstor.org/pss/115394.  ^ Lord Rayleigh; Ramsay, William (1895). "VI. Argon: A New Constituent of the Atmosphere". Philosophical Transactions of the Royal Society of London. A 186: 187. doi:10.1098/rsta.1895.0006. http://www.jstor.org/pss/90645.  ^ William Ramsay. "Nobel Lecture in Chemistry, 1904". http://nobelprize.org/nobel_prizes/chemistry/laureates/1904/ramsay-lecture.html.  ^ "About Argon, the Inert; The New Element Supposedly Found in the Atmosphere". The New York Times. 1895-03-03. http://query.nytimes.com/gst/abstract.html?res=9B04E3D61139E033A25750C0A9659C94649ED7CF. Retrieved 2009-02-01.  ^ Holden, Norman E. (12). "History of the Origin of the Chemical Elements and Their Discoverers". National Nuclear Data Center (NNDC). http://www.nndc.bnl.gov/content/elements.html.  ^ "Argon, Ar". http://elements.etacude.com/Ar.php. Retrieved 2007-03-08.  ^ a b "40Ar/39Ar dating and errors". Archived from the original on May 9, 2007. http://web.archive.org/web/20070509023017/http://www.geoberg.de/text/geology/07011601.php. Retrieved 2007-03-07.  ^ Lodders, Katharina (2008). "the solar argon abundance". The Astrophysical Journal 674: 607. doi:10.1086/524725. http://arxiv.org/abs/0710.4523.  ^ "Seeing, touching and smelling the extraordinarily Earth-like world of Titan". European Space Agency. 21. http://www.esa.int/esaCP/SEMHB881Y3E_index_0.html.  ^ Bartlett, Neil. "The Noble Gases". Chemical & Engineering News (2003). http://pubs.acs.org/cen/80th/noblegases.html.  ^ Lockyear, Jessica F. et al.; Douglas, Kevin; Price, Stephen D.; Karwowska, MałGorzata; Fijalkowski, Karol J.; Grochala, Wojciech; Remeš, Marek; Roithová, Jana et al. (2010). "Generation of the ArCF22+ Dication". J. Phys. Chem. Letts. 1: 358. doi:10.1021/jz900274p.  ^ "Periodic Table of Elements: Argon – Ar". Environmentalchemistry.com. http://environmentalchemistry.com/yogi/periodic/Ar.html. Retrieved 2008-09-12.  ^ Fletcher, D. L.. "Symposium: Recent Advances in Poultry Slaughter Technology Slaughter Technology". http://ps.fass.org/cgi/reprint/78/2/277.pdf. Retrieved 2010-01-01.  ^ "Measurement of scintillation efficiency for nuclear recoils in liquid argon". http://arxiv.org/abs/1004.0373.  ^ Zawalick, Steven Scott "Method for preserving an oxygen sensitive liquid product" U.S. Patent 6,629,402 Issue date: October 7, 2003 ^ "Schedule for Renovation of the National Archives Building". http://www.archives.gov/press/press-kits/charters.html#pressrelaese1. Retrieved 2009-07-07.  ^ "Fatal Gas Embolism Caused by Overpressurization during Laparoscopic Use of Argon Enhanced Coagulation". MDSR. 24. http://www.mdsr.ecri.org/summary/detail.aspx?doc_id=8248.  ^ Fujimoto, James; Rox Anderson, R. (2006). "Tissue Optics, Laser-Tissue Interaction, and Tissue Engineering" (pdf). Biomedical Optics. pp. 77–88. http://www.spie.org/Conferences/Programs/06/pw/BiOSAbstracts.pdf. Retrieved 2007-03-08.  ^ Pilmanis Andrew A, Balldin UI, Webb James T, Krause KM (December 2003). "Staged decompression to 3.5 psi using argon-oxygen and 100% oxygen breathing mixtures". Aviation, Space, Environmental Medicine 74 (12): 1243–50. PMID 14692466.  ^ "Energy-Efficient Windows". FineHomebuilding.com. http://www.finehomebuilding.com/how-to/articles/understanding-energy-efficient-windows.aspx. Retrieved 2009-08-01.  ^ Nuckols ML, Giblo J, Wood-Putnam JL. (September 15–18, 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). http://archive.rubicon-foundation.org/7962. Retrieved 2009-03-02.  ^ "Description of Aim-9 Operation". planken.org. http://home.wanadoo.nl/tcc/rnlaf/aim9.html. Retrieved 2009-02-01.  ^ "Welder's Helper Asphyxiated in Argon-Inerted Pipe -- Alaska (FACE AK-94-012)". State of Alaska Department of Public Health. 1994-06-23. http://www.cdc.gov/niosh/face/stateface/ak/94ak012.html. Retrieved 2011-01-29.  Further reading Book: Argon Wikipedia Books are collections of articles that can be downloaded or ordered in print. Wikimedia Commons has media related to: 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.; .; 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 ed.). Boca Raton, Florida: CRC Press. 2005.  External links Look up argon in Wiktionary, the free dictionary. WebElements.com – Argon Diving applications: Why Argon? Argon Ar Properties, Uses, Applications v · d · e Periodic table H   He Li Be   B C N O F Ne Na Mg   Al Si P S Cl Ar K Ca   Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr   Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo Alkali metals Alkaline earth metals Lanthanides Actinides Transition metals Other metals Metalloids Other nonmetals Halogens Noble gases Unknown chem. properties Large version