strontium [from Strontian, a Scottish town], a metallic chemical element; symbol Sr; at. no. 38; at. wt. 87.62; m.p. 769°C;; b.p. 1,384°C;; sp. gr. 2.6 at 20°C;; valence +2. Strontium is a soft, silver-yellow metal with three allotropic crystalline forms (see allotropy). It is an alkaline-earth metal; in its physical and chemical properties it resembles calcium and barium, the elements above and below it in Group 2 of the periodic table. Since strontium reacts vigorously with water and quickly tarnishes in air, it must be stored out of contact with air and water. Among its compounds are the oxide strontia, SrO; peroxide, SrO₂ ; hydroxide, Sr(OH)₂ ; nitrate, Sr(NO₃)₂ ; the carbonate strontianite, SrCO₃ ; the sulfate celestite, SrSO₄ ; carbide, SrC₂ ; and halides, SrBr₂, SrCl₂, SrF₂, and SrI₂. Celestite and strontianite are the chief ores of strontium. The metal may be prepared by electrolysis of fused strontium chloride; small amounts of the metal are used in semiconductor devices. Although strontium has uses similar to those of calcium and barium, it is rarely employed because of its higher cost. Principal uses of strontium compounds are in pyrotechnics (chiefly the nitrate) and in greases (the hydroxide). In fireworks and signal flares strontium compounds add a bright red or crimson color to the flame. Naturally occurring strontium is a mixture of four stable isotopes. Twelve unstable isotopes exist; the most stable of these is the radioactive isotope strontium-90 (half-life 28.1 years), which is the chief immediate hazard in fallout. As a result of atmospheric nuclear tests, strontium-90 is dispersed in varying concentrations throughout the earth's atmosphere and soil. Because of its chemical similarity to calcium, it is readily taken up in the tissues of plants and animals; it may enter the human food supply, mainly in milk. It is particularly dangerous for growing children as it is easily deposited in the bones and is believed to induce bone cancer and leukemia. Strontium-90 also has some uses in luminous signs and in nuclear batteries. Strontium was first recognized as distinct from barium in 1790 by A. Crawford in a sample of its carbonate from a mine near Strontian, Scotland; his finding was later confirmed by T. C. Hope, M. H. Klaproth, and others. It was first isolated by electrolysis in 1808 by Humphry Davy.

Atomic Number:	Atomic Number: 38
Atomic Symbol:	Atomic Symbol: Sr
	Name of Element: Strontium
Atomic Weight:	Atomic Weight: 87.62
Electron Configuration:	Electron Configuration: 2 · 8 · 188 · 2

Chemical element, one of the alkaline earth metals, chemical symbol Sr, atomic number 38. A soft metal, it has a silvery lustre when freshly cut but reacts rapidly with air. In both the metal and the compounds (in which it has valence 2), strontium resembles calcium and barium so closely that it has few uses that the other two elements cannot supply more cheaply. The nitrate and chlorate, very volatile, give off brilliant crimson flames and are used in flares, fireworks, and tracer bullets. The radioactive isotope strontium-90 (see radioactivity), produced in nuclear explosions, is the principal health hazard in fallout; it can replace some of the calcium in foods, concentrate in bones and teeth, and cause radiation injury.

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Method of estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium-87 formed by the decay of the unstable isotope rubidium-87 that was present in the rock at the time of its formation. The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 48.8 billion years. Seealso dating.

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