strontium (strŏnˈshēəm) [key] [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.