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Strontium
Strontium is a chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when it is exposed to air. Strontium has physical and chemical properties similar to those of its two neighbors calcium and barium. It occurs naturally in the minerals celestine, putnisite and strontianite. While natural strontium is stable, the synthetic 90Sr isotope is present in radioactive fallout and has a half-life of 28.90 years.
Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was discovered in 1790 by Adair Crawford and William Cruickshank. The production of sugar from sugar beet was in the 19th century its largest application (see strontian process). At the peak of production of television cathode ray tubes, up to 75 percent of U.S. strontium consumption was used to make the faceplate glass.〔(【引用サイトリンク】title=Mineral Resource of the Month: Strontium )〕 With the displacement of cathode ray tubes by other display methods in television sets consumption of strontium has dramatically declined.〔
==Characteristics==
Strontium is a grey, silvery metal that is softer than calcium and even more reactive toward water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C, at room temperature it will only form the oxide spontaneously.〔 Three allotropes of metallic strontium exist, with transition points at 235 and 540 °C.
Because of its extreme reactivity with oxygen and water, this element occurs naturally only in compounds with other elements, such as in the minerals strontianite and celestite. It is kept under a liquid hydrocarbon such as mineral oil or kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal is pyrophoric meaning it will ignite spontaneously in air at room temperature. Volatile strontium salts impart a bright red color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four stable isotopes.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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