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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.〔In older versions of the periodic table, the noble gases were identified as Group VIIIA or as Group 0. See Group (periodic table).〕 Argon is the third most common gas in the Earth's atmosphere, at 0.93% (9,300 ppm), making it approximately 23.7 times as abundant as the next most common atmospheric gas, carbon dioxide (390 ppm), and more than 500 times as abundant as the next most common noble gas, neon (18 ppm). 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.〔http://www.ptable.com/#Property/Abundance/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. ==Characteristics== 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.〔(Material Safety Data Sheet Gaseous Argon ), Universal Industrial Gases, Inc. Retrieved 14 October 2013.〕 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 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「argon」の詳細全文を読む スポンサード リンク
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