Strontium
Strontium is a chemical element in the periodic table that has the symbol Sr and the atomic number 38.
- Atomic number: 38
- Atomic weight: 87.62
- Density: 2.6 g/cm3
- Melting point: 800°C
- Boiling point: 1150°C
- Electron configuration: [Kr]5s2
History
Adair Crawford recognized the mineral strontianite, named after the Scottish town of Strontian, as differing from other barium minerals in 1790 but metallic strontium was first isolated by Davey in 1808 using electrolysis.
Forms
Strontium is found chiefly as the form of the sulfate mineral celestite (SrSO4) and the carbonate strontianite (SrCO3). The metal can be prepared by electrolysis of the melted chloride mixed with potassium chloride, or is made by reducing strontium oxide with aluminum 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.
Properties
Strontium is a bright silvery metal that is softer than calcium and even more reactive in water; strontium will decompose 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 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. Volatile strontium salts impart a beautiful crimson 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.
Isotopes
The alkali earth metal strontium has four stable, naturally occurring isotopes: 84Sr (0.56%), 86Sr (9.86%), 81Sr (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 48,800,000 years. Thus, there are two sources of 87Sr in any material: that formed during primordial nucleo-synthesis 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. Because strontium has an atomic radius similar to that of Ca, it readily substitutes for Ca in minerals. 87Sr/86Sr ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0.
Sixteen unstable isotopes are known to exist. Of greatest importance is 90Sr with a half-life of 29 years. It is a product of nuclear fallout and presents a health problem since it substitutes for calcium in bone, preventing expulsion from the body. This isotope is one of the best long-lived high-energy beta emitters known, and is used in SNAP (Systems for Nuclear Auxilliary Power) devices. These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, etc., where a lightweight, long-lived, nuclear-electric power source is needed.
Uses
The major use for strontium at present is in producing glass for color television cathode ray tubes. It has also found use in producing ferrite magnets and in refining zinc. Strontium titanate is an interesting optical material as it has an extremely high refractive index and an optical dispersion greater than that of diamond. It has been used as a gemstone, but is very soft and so scratches easily. It does not occur naturally.
Cost
Strontium metal (98% pure) in January 1990 cost about $5/oz. The largest commercially exploited deposits are found in England.
External links
Los Alamos National Laboratory - Strontium