|Name, Symbol, Number||strontium, Sr, 38|
|Chemical series||alkaline earth metals|
|Group, Period, Block||2, 5, s|
|Appearance||silvery white metallic
|Atomic mass||87.62(1) g/mol|
|Electron configuration||[Kr] 5s2|
|Electrons per shell||2, 8, 18, 8, 2|
|Density (near r.t.)||2.64 g/cm³|
|Liquid density at m.p.||6.980 g/cm³|
|Melting point||1050 K
(777 °C, 1431 °F)
|Boiling point||1655 K
(1382 °C, 2520 °F)
|Heat of fusion||7.43 kJ/mol|
|Heat of vaporization||136.9 kJ/mol|
|Heat capacity||(25 °C) 26.4 J/(mol·K)|
|Crystal structure||cubic face centered|
(strongly basic oxide)
|Electronegativity||0.95 (Pauling scale)|
|1st: 549.5 kJ/mol|
|2nd: 1064.2 kJ/mol|
|3rd: 4138 kJ/mol|
|Atomic radius||200 pm|
|Atomic radius (calc.)||219 pm|
|Covalent radius||192 pm|
|Electrical resistivity||(20 °C) 132 nΩ·m|
|Thermal conductivity||(300 K) 35.4 W/(m·K)|
|Thermal expansion||(25 °C) 22.5 µm/(m·K)|
|Shear modulus||6.1 GPa|
|CAS registry number||7440-24-6|
Strontium (chemical symbol Sr, atomic number 38) is a soft, silvery white metallic element that occurs naturally in the minerals celestite and strontianite. Highly reactive chemically, it is converted to its yellowish oxide when exposed to air. Classified as an alkaline earth metal, it reacts with water to produce the alkali strontium hydroxide. The properties of strontium are closest to those of calcium, and it can replace calcium in bone tissue.
When burned, strontium salts produce an attractive red color, and they are therefore used in pyrotechnic displays and aerosol paints. In addition, strontium is widely used in the glass screens of color television sets. Strontium titanate has a various applications in optics, strontium aluminate is used as a phosphorescent material, and strontium chloride may be added to toothpastes for people with sensitive teeth. The radioactive isotopes and 90Sr are used in cancer therapy, and the latter may also be used in devices that generate electrical power for spacecraft and remote weather stations.
Although the stable isotopes of strontium are not a threat to human health, unrestricted exposure to the radioactive isotope 90Sr—which is present in nuclear fallout—can lead to various bone disorders and bone cancer. On the other hand, controlled doses of 90Sr are useful in cancer therapy. This isotope may also be used in devices that generate electrical power for spacecraft and remote weather stations.
Strontium occurs commonly in nature amd has been estimated to be the fifteenth most abundant element on Earth, averaging 0.034 percent of all igneous rock. Given its extreme reactivity, its natural occurrence is only in the form of compounds with other elements. Its chief minerals are celestite (strontium sulfate, SrSO4) and strontianite (strontium carbonate, SrCO3). The largest commercially exploited deposits are found in England.
Of the two minerals, celestite occurs in sufficient amounts in sedimentary deposits to make development of mining facilities attractive. It would be more useful to mine strontianite because strontium is used more often in the carbonate form, but there are relatively few known deposits suitable for development.
In 1790, while examining samples of the barium mineral witherite (barium carbonate, BaCO3), Adair Crawford found the samples to contain a previously unrecorded mineral. The new mineral was named strontianite, after the Scottish village of Strontian. The element strontium itself was discovered in 1798, and metallic strontium was first isolated by Sir Humphry Davy in 1808, by the method of electrolysis.
As a member of the series of alkaline earth metals, strontium lies in group 2 (former group 2A) of the periodic table, between calcium and barium. In addition, it is placed in period 5, between rubidium and yttrium. As its atomic radius is similar to that of calcium, it readily substitutes for calcium in minerals.
Freshly prepared strontium has a bright silvery color, but on exposure to air it forms the yellow oxide. It is softer than calcium and even more reactive in water. On contact with water, strontium reacts to produce strontium hydroxide and hydrogen gas. Three allotropes of strontium are known, with transition points at 235 and 540 °C.
Strontium normally does not react with nitrogen below 380 °C, and forms only the oxide at room temperature. When finely powdered, however, the metal ignites spontaneously in air to produce both strontium oxide and strontium nitride. To prevent it from reacting with air or water, strontium should be stored under kerosene.
In nature, strontium occurs as four stable isotopes: 84Sr (0.56 percent), 86Sr (9.86 percent), 87Sr (7.0 percent), and 88Sr (82.58 percent). Of these, only 87Sr is radiogenic—it is produced by the decay of a radioactive isotope of rubidium, 87Rb, which has a half-life of 4.88 × 1010 years. It is thought that 87Sr was also produced during "primordial nucleosynthesis" (the early stages of the Big Bang), along with the isotopes 84Sr, 86Sr, and 88Sr. The ratio 87Sr/86Sr is the parameter typically reported in geologic investigations. In minerals and rocks, the ratios range from about 0.7 to greater than 4.0.
In addition to the stable isotopes, 16 unstable isotopes of strontium are known. Among them, the most significant one is 90Sr, with a half-life of 28.78 years. As a byproduct of nuclear fission, it is present in nuclear fallout and presents a health problem because it substitutes for calcium in bone, preventing expulsion from the body. The 1986 Chernobyl nuclear accident contaminated a vast area with 90Sr.
Pure strontium is extremely reactive, and finely divided strontium burns spontaneously. It is therefore considered a fire hazard.
The human body absorbs strontium as if it were calcium. These two elements are chemically so similar that the stable forms of strontium do not pose a significant health threat. By contrast, the radioactive isotope 90Sr can lead to various bone disorders and diseases, including bone cancer. The "strontium unit" is used in measuring radioactivity from absorbed 90Sr.
All links retrieved October 23, 2015.
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