|silvery white metallic
|Name, symbol, number||strontium, Sr, 38|
|Element category||alkaline earth metal|
|Group, period, block||2, 5, s|
|Standard atomic weight||87.62 g·mol−1|
|Electron configuration||[Kr] 5s2|
|Electrons per shell||2, 8, 18, 8, 2 (Image)|
|Density (near r.t.)||2.64 g·cm−3|
|Liquid density at m.p.||2.375 g·cm−3|
|Melting point||1050 K, 777 °C, 1431 °F|
|Boiling point||1655 K, 1382 °C, 2520 °F|
|Heat of fusion||7.43 kJ·mol−1|
|Heat of vaporization||136.9 kJ·mol−1|
|Specific heat capacity||(25 °C) 26.4 J·mol−1·K−1|
|Oxidation states||2, 1 (strongly basic oxide)|
|Electronegativity||0.95 (Pauling scale)|
|Ionization energies||1st: 549.5 kJ·mol−1|
|2nd: 1064.2 kJ·mol−1|
|3rd: 4138 kJ·mol−1|
|Atomic radius||215 pm|
|Covalent radius||195±10 pm|
|Van der Waals radius||249 pm|
|Crystal structure||cubic face-centered|
|Electrical resistivity||(20 °C) 132 nΩ·m|
|Thermal conductivity||(300 K) 35.4 W·m−1·K−1|
|Thermal expansion||(25 °C) 22.5 µm·m−1·K−1|
|Shear modulus||6.1 GPa|
|CAS registry number||7440-24-6|
|Most stable isotopes|
|Main article: Isotopes of strontium|
Strontium (pronounced /ˈstrɒnʃiəm/ STRON-shee-əm, /ˈstrɒntiəm/ STRON-tee-əm, or /ˈstrɒnʃəm/ STRON-shəm) is a chemical element with the symbol Sr and the 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 exposed to air. It occurs naturally in the minerals celestine and strontianite. The 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 first discovered.
Strontium is a grey/silvery metal that is softer than calcium and even more reactive in water, with which strontium 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 it will only form the oxide spontaneously at room temperature. It should be 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 will ignite spontaneously in air at room temperature. Volatile strontium salts impart a crimson color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four radiostable isotopes.
Strontium has four stable, naturally occurring isotopes: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). Only 87Sr is radiogenic; it is produced by decay from the radioactive alkali metal 87Rb, which has a half-life of 4.88 × 1010 years. Thus, there are two sources of 87Sr in any material: that formed in stars along with 84Sr, 86Sr and 88Sr, as well as that formed by radioactive decay of 87Rb. The ratio 87Sr/86Sr is the parameter typically reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because strontium has an atomic radius similar to that of calcium, it readily substitutes for Ca in minerals.
Strontium is named after the Scottish village of Strontian, having been discovered in the ores taken from the lead mines. In 1790, Adair Crawford, a physician engaged in the preparation of barium, recognised that the Strontian ores exhibited different properties to those normally seen with other "heavy spars" sources. This allowed him to conclude "... it is probable indeed, that the scotch mineral is a new species of earth which has not hitherto been sufficiently examined". The new mineral was named strontites in 1793 by Thomas Charles Hope, a professor of medicine at the University of Glasgow. He confirmed the earlier work of Crawford and recounted: " ... Considering it a peculiar earth I thought it necessary to give it an name. I have called it Strontites, from the place it was found; a mode of derivation in my opinion, fully as proper as any quality it may possess, which is the present fashion". The element was eventually isolated by Sir Humphry Davy in 1808 by the electrolysis of a mixture containing strontium chloride and mercuric oxide, and announced by him in a lecture to the Royal Society on 30 June 1808. In keeping with the naming of the other alkaline earths, he changed the name to strontium.
Strontium commonly occurs in nature, the 15th most abundant element on earth, averaging 0.034% of all igneous rock and is found chiefly as the form of the sulfate mineral celestite (SrSO4) and the carbonate strontianite (SrCO3). Of the two, celestite occurs much more frequently in sedimentary deposits of sufficient size to make development of mining facilities attractive. Strontianite would be the more useful of the two common minerals because strontium is used most often in the carbonate form, but few deposits have been discovered that are suitable for development. The metal can be prepared by electrolysis of melted strontium chloride mixed with potassium chloride:
Alternatively it is made by reducing strontium oxide with aluminium in a vacuum at a temperature at which strontium distills off. Three allotropes of the metal exist, with transition points at 235 and 540 °C. The largest commercially exploited deposits are found in England.
As a pure metal strontium is used in strontium 90%-aluminium 10% alloys of an eutectic composition for the modification of aluminium-silicon casting alloys. Strontium is 2% by weight of AJ62 alloy, a durable, creep-resistant magnesium alloy used in car and motorcycle engines by BMW.
The primary use for strontium compounds is in glass for colour television cathode ray tubes to prevent X-ray emission. All parts of the CRT tube have to absorb X-rays. In the neck and the funnel of the tube lead glass is used for this purpose, but this type of glass shows a browning effect due to the interaction of the X-rays with the glass. Therefore the front panel has to use a different glass mixture, in which strontium and barium are the X-ray absorbing materials. The average values for the class mixture determined for a recycling studie in 2005 is 8.5% strontium oxide and 10% barium oxide.
Scientific (low quantity) use :
Uses of radioactive strontium isotopes :
Strontium isotopes are measured for various reasons :
The human body absorbs strontium as if it were calcium. Due to the elements being sufficiently similar chemically, the stable forms of strontium might not pose a significant health threat—in fact, the levels found naturally may actually be beneficial (see below) -- but the radioactive 90Sr can lead to various bone disorders and diseases, including bone cancer. The strontium unit is used in measuring radioactivity from absorbed 90Sr.
A recent in-vitro study conducted the NY College of Dental Sciences using strontium on osteoblasts showed marked improvement on bone-building osteoblasts.
The drug strontium ranelate, made by combining strontium with ranelic acid, was found to aid bone growth, increase bone density, and lessen vertebral, peripheral and hip fractures. Women receiving the drug showed a 12.7% increase in bone density. Women receiving a placebo had a 1.6% decrease. Half the increase in bone density (measured by x-ray densitometry) is attributed to the higher atomic weight of Sr compared with calcium, whereas the other half a true increase in bone mass. Strontium ranelate is registered as a prescription drug in Europe and many countries worldwide. It needs to be prescribed by a doctor, delivered by a pharmacist, and requires strict medical supervision. Currently (early 2007), it is not available in Canada or the United States.
There is a long history of medical research regarding strontium's benefits, beginning in the 1950s. Studies indicate a lack of undesirable side-effects. Several other salts of strontium such as strontium citrate or strontium carbonate are often presented as natural therapies and sold at a dose that is several hundred times higher than the usual strontium intake. Such compounds are sold in the United States under the Dietary Supplements Health and Education Act of 1994. Their long-term safety and efficacy have never been evaluated on humans using large-scale medical trials.
STRONTIUM [[[Symbol]] Sr, atomic weight 87.62 (0 = 16)], a metallic chemical element belonging to the alkaline earth group. It is found in small quantities very widely distributed in various rocks and soils, and in mineral waters; its chief sources are the minerals strontianite, celestine and barytocelestine. The metal was detected in the mineral strontianite, found at Strontian in Argyllshire, by Cruikshank in 1787, and by Crawford in 17 9 0; and the discovery was confirmed by Hope in 1792 and by Klaproth in 1793. The metal was isolated in 1807 by Sir H. Davy by electrolysing the moist hydroxide or chloride, and has been obtained by A. Guntz and Roederer (Comptes rendus, 1906, 142, p. 400) by heating the hydride in a vacuum to 1000. By electrolysing an aqueous solution of the chloride with a mercury cathode, a liquid and a solid amalgam, SrHgn, are obtained; the latter on heating gives a mixture of Sr 2 Hg 5 and SrHg 5, and on distillation an amalgam passes over, and not the metal. It is a silver-white ductile metal (of specific gravity 2.54) which melts at 8000. It oxidizes rapidly when exposed to air, and burns when heated in air, oxygen, chlorine, bromine or sulphur vapour. With dry ammonia at 60° the metal forms strontium ammonium, which slowly decomposes in a vacuum at 20° giving Sr(NH 3) 2; with carbon monoxide it gives Sr(CO) 2; with oxygen it forms the monoxide and peroxide, and with nitric oxide it gives the hyponitrite (Roederer, Bull. soc. chim., 1906 [iii.], 35, P. 715).
The hydride, SrH 2, was obtained by Guntz on heating strontium amalgam in a current of hydrogen. It is a white solid, which readily decomposes water in the cold and behaves as a strong reducing agent. It dissociates when heated to a high temperature and is not affected by oxygen. The monoxide or strontia, Sr(); is formed by strongly heating the nitrate, or commercially by heating the sulphide or carbonate in superheated steam (at about 500-600° C.). It is a white amorphous powder which resembles lime in its general character. By heating the amorphous form in the electric furnace H. Moissan succeeded in obtaining a crystalline variety. The amorphous form readily slakes with water, and the aqueous solution yields a crystalline hydrated hydroxide approximating in composition to Sr(OH) 2.8H 2 O or Sr(OH) 2.9H 2 O, which on standing in vacuo loses some of its water of crystallization, leaving the monohydrated hydroxide, Sr(OH) 2 H 2 O. The ordinary hydrated variety forms quadratic crystals and behaves as a strong base. It is used in the extraction of sugar from molasses, since it combines with the sugar to form a soluble saccharate, which is removed and then decomposed by carbon dioxide. A hydrated dioxide, approximating in composition to SrO 2.8H 2 O, is formed as a crystalline precipitate when hydrogen peroxide is added to an aqueous solution of strontium hydroxide.
Strontium fluoride, SrF 2, is obtained by the action of hydrofluoric acid on the carbonate, or by the addition of potassium fluoride to strontium chloride solution. It may be obtained crystalline by fusing the anhydrous chloride with a large excess of potassium hydrogen fluoride or by heating the amorphous variety to redness with an excess of an alkaline chloride. Strontium chloride, SrC1 2.6H 2 O, is obtained by dissolving the carbonate in hydrochloric acid, or by fusing the carbonate with calcium chloride and extracting the melt with water. It crystallizes in small colourless needles and is easily soluble in water; the concentrated aqueous solution dissolves bromine and iodine readily. By concentrating the aqueous solution between 90-130° C., or by passing hydrochloric acid gas into a saturated aqueous solution, a second hydrated form of composition, SrC1 2.2H 2 O, is obtained. The anhydrous chloride is formed by heating strontium or its monoxide in chlorine, or by heating the hydrated chloride in a current of hydrochloric acid gas. It is a white solid, which combines with gaseous ammonia to form SrC1 2.8NH 3, and when heated in superheated steam it decomposes with evolution of hydrochloric acid.
Strontium sulphide, SrS, is formed when the carbonate is heated to redness in a stream of sulphuretted hydrogen. It phosphoresces very slightly when pure. Strontium sulphate, SrSO 4, found in the mineral kingdom as celestine, is formed when sulphuric acid or a soluble sulphate is added to a solution of a strontium salt. It is a colourless, amorphous solid, which is almost insoluble in water, its solubility diminishing with increasing temperature; it is appreciably soluble in concentrated sulphuric acid. When boiled with alkaline carbonates it is converted into strontium carbonate.
Strontium nitride, Sr 3 N 2, is formed when strontium amalgam is heated to redness in a stream of nitrogen or by igniting the oxide with magnesium (H. R. Ellis, Chem. News, 1909, 99, p. 4). It is readily decomposed by water, with liberation of ammonia. Strontium nitrate, Sr(N03)2, is obtained by dissolving the carbonate in dilute nitric acid. It crystallizes from water (in which it is very soluble) in monoclinic prisms which approximate in composition to Sr(N03)2.4H20 or Sr(N03)2.5H20. When heated it fuses in its own water of crystallization and becomes anhydrous at 110° C. It is used in pyrotechny for the manufacture of red-fire. A strontium boride, SrB6, was obtained as a black crystalline powder by H. Moissan and P. Williams (Comptes rendus, 1897, 12 3, p. 6 33) by reducing the borate with aluminium in the electric furnace.
Strontium carbide, SrC2, is obtained by heating strontium carbonate with carbon in the electric furnace. It resembles calcium carbide, decomposing rapidly with water, giving acetylene. Strontium carbonate, SrCO 3, found in the mineral kingdom as strontianite, is formed when a solution of a carbonate is added to one of a strontium salt. It is an amorphous solid, insoluble in water, but its solubility is increased in the presence of ammonium nitrate. It loses carbon dioxide when heated to high temperature.
Strontium salts may be recognized by the characteristic crimson colour they impart to the flame of the Bunsen burner and by the precipitation of the insoluble sulphate. On the preparation of pure strontium salts, see Adrian and Bougarel, Journ. pharm. chem., 18 9 2 (5), p. 345; and S. P. L. Soerenoen, Zeit. anorg. chem., 1895, II, p. 305. Recent determinations of the atomic weight of strontium are due to T. W. Richards (Zeit. anorg. Chem-., 1905, 47, p. 1 45), who, by estimating the ratios of strontium bromide and chloride to silver, obtained the values 87.663 and 87.661.
Strontium is a chemical element. It has the chemical symbol Sr. It has the atomic number 38. It is a metal. The colour of the metal is silver-white or yellow-silver. The metal is soft. In chemistry it is placed in a group of metal elements named the alkaline earth metals. Strontium has a high chemical reactivity. The metal turns yellow when exposed to air.