2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||strontium, Sr, 38|
|Element category||alkaline earth metals|
|Group, Period, Block||2, 5, s|
|Appearance||silvery white metallic
|Standard atomic weight||87.62 (1) g·mol−1|
|Electron configuration||[Kr] 5s2|
|Electrons per shell||2, 8, 18, 8, 2|
|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|
|Crystal structure||cubic face centered|
|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||200 pm|
|Atomic radius (calc.)||219 pm|
|Covalent radius||192 pm|
|Electrical resistivity||(20 °C) 132 n Ω·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|
Strontium (pronounced /ˈstrɒntiəm/) 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.
Due to its extreme reactivity to air, this element occurs naturally only in compounds with other elements, as in the minerals strontianite and celestite.
Strontium is a bright 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 kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish colour with the formation of the oxide. Finely powdered strontium metal will ignite spontaneously in air. Volatile strontium salts impart a crimson colour to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four stable isotopes.
As a pure metal strontium is being used in strontium 90%-aluminium 10% alloys of an eutectic composition for the modification of aluminium-silicon casting alloys. The primary use for strontium compounds is in glass for colour television cathode ray tubes to prevent X-ray emission.
- 89Sr is the active ingredient in Metastron, a radiopharmaceutical used for bone pain secondary to metastatic prostate cancer. The strontium acts like calcium and is preferentially incorporated into bone at sites of increased osteogenesis. This localization focuses the radiation exposure on the cancerous lesion.
- 90Sr has been used as a power source for radioisotope thermoelectric generators (RTGs). 90Sr produces about 0.93 watts of heat per gram (it is lower for the grade of 90Sr used in RTGs, which is strontium fluoride). However, 90Sr has a lifetime approximately 3 times shorter and has a lower density than 238Pu, another RTG fuel. The main advantage of 90Sr is that it is cheaper than 238Pu and is found in nuclear waste.
- 90Sr is also used in cancer therapy. Its beta emission and long half-life is ideal for superficial radiotherapy.
- Strontium is one of the constituents of AJ62 alloy, a durable magnesium alloy used in car and motorcycle engines by BMW.
- Since Strontium is so similar to calcium, it is incorporated in the bone. All four isotopes are incorporated, in roughly similar proportions as they are found in nature (please see below). However the actual distribution of the isotopes tends to vary greatly from one geographical location to another. Thus analyzing the bone of an individual can help determine the region it came from. This approach helps to identify the ancient migration patterns as well as the origin of commingled human remains in battlefield burial sites. Strontium thus helps forensic scientists too.
87Sr/86Sr ratios are commonly used to determine the likely provenance areas of sediment in natural systems, especially in marine and fluvial environments. Dasch (1969) showed that surface sediments of Atlantic displayed 87Sr/86Sr ratios that could be regarded as bulk averages of the 87Sr/86Sr ratios of geological terranes from adjacent landmasses. A good example of a fluvial-marine system to which Sr isotope provenance studies have been successfully employed is the River Nile-Mediterranean system (Krom et al, 1999; Krom et al, 2002; Talbot et al. 2000). Due to the differing ages of the rocks that constitute the majority of the Blue and White Nile catchment areas the changing provenance of sediment reaching the River Nile delta and East Mediterranean Sea can be discerned through Sr isotopic studies. Such changes are climatically controlled in the Late Quaternary.
More recently, 87Sr/86Sr ratios have also been used to determine the source of ancient archaeological materials such as timbers and corn in Chaco Canyon, New Mexico (English et al, 2001; Benson et al, 2003). 87Sr/86Sr ratios in teeth may also be used to track animal migrations (Barnett-Johnson, 2007; Porder et al., 2003) or in criminal forensics.
Strontium atoms are used in an experimental atomic clock with record-setting accuracy.
- Ferrite magnets and refining zinc.
- Strontium titanate has an extremely high refractive index and an optical dispersion greater than that of diamond, making it useful in a variety of optics applications. This quality has also led to it being cut into gemstones, in particular as a diamond simulant. However, it is very soft and easily scratches so it is rarely used.
- Strontium carbonate, Strontium nitrate, and Strontium sulfate are commonly used in fireworks for red colour.
- Strontium aluminate is used as a bright phosphor with long persistence of phosphorescence.
- Strontium chloride is sometimes used in toothpastes for sensitive teeth. One popular brand includes 10% total strontium chloride hexahydrate by weight.
- Strontium oxide is sometimes used to improve the quality of some pottery glazes.
- Strontium is also commonly used in aerosol paint, such as the Spanish Montana (Montana Hardcore). This is one of the most likely sources of exposure to the public.
- Strontium ranelate is used in the treatment of osteoporosis. It is a prescription drug in the EU and UK, but not in the USA, and is marketed by Servier, a French drug house, as Protelos.
The mineral strontianite is named after the Scottish village of Strontian, having been discovered in the lead mines there in 1787. Adair Crawford recognized it as differing from other barium minerals in 1790. Strontium itself was discovered in 1798 by Thomas Charles Hope, and metallic strontium was first isolated by Sir Humphry Davy in 1808 using electrolysis and announced by him in a lecture to the Royal Society on 30th June 1808.
Strontium was among the radioactive materials released by the 1957 Windscale fire.
In 2005, China was the top producer of strontium with almost two-thirds world share followed by Spain and Mexico, reports the British Geological Survey.
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:
- Sr2+ + 2 e- → Sr
- 2 Cl- → Cl2 (g) + 2 e-
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.
The alkali earth metal 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 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; 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.
Sixteen unstable isotopes are known to exist. Of greatest importance are 90Sr with a half-life of 28.78 years and 89Sr with a half-life of 50.5 days.
- 90Sr is a by-product of nuclear fission which is found in 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 Auxiliary 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 required. The 1986 Chernobyl nuclear accident contaminated a vast area with 90Sr. 90Sr confined inside a concave silver plaque is also used for the medical treatment of a resected pterygium.
- 89Sr is a short-lived artificial radioisotope which provides a health benefit since it substitutes for calcium in bone. In circumstances where cancer patients have widespread and painful bony metastases (secondaries), the administration of 89Sr results in the delivery of radioactive emissions ( beta particles in this case) directly to the area of bony problem (where calcium turnover is greatest). The 89Sr is manufactured as the chloride salt (which is soluble), and when dissolved in normal saline can be injected intravenously. Typically, cancer patients will be treated with a dose of 150 MBq. The patient needs to take precautions following this because their urine becomes contaminated with radioactivity, so they need to sit to urinate and double flush the toilet. The beta particles travel about 3.5mm in bone (energy 0.583 MeV) and 6.5mm in tissue, so there is no requirement to isolate patients who have been treated except to say they should not have any one (especially young children) sitting in their laps for 10-40 days. The variation in time results from the variable clearing time for 89Sr which depends on renal function and the number of bony metastases. With a lot of bony metastases, the entire 89Sr dose can be taken up into bone and so the entire radioactivity is retained to decay over a 50.5 day half-life. However, where there are few bony metastases, the large proportion of 89Sr not taken up by the bone will be filtered by the kidney, so that the effective half-life (a combination of the physical and biological half-life) will be much shorter.
In its pure form strontium is extremely reactive with air and spontaneously combusts. It is therefore considered to be a fire hazard.
Effect on the human body
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, 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 ( http://iadr.confex.com/iadr/2007orleans/techprogram/abstract_89231.htm.)
An innovative drug made by combining strontium with ranelic acid has aided in bone growth, boosted bone density and lessened 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. It means that strontium ranelate creates new, stronger bone. Strontium ranelate (marketed under the trade names Protelos, Osseor, Protos, Bivalos, Protaxos, Ossum) is registered for treatment of osteoporosis in many countries all over the world. Strontium ranelate has been shown to strengthen bones, according to presentations given at the IOF World Congress on Osteoporosis, in June of 2006. It also reduced bone resorption.
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.
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. Despite the lack of strontium deficit referenced in the medical literature and the lack of information about possible toxicity of strontium supplementation, such compounds can still be sold in the United States under the Dietary Supplements Health and Education Act of 1994. However, their long-term safety and efficacy have never been evaluated on humans using large-scale medical trials. Such compounds should not be administered to humans before further studies are conducted.
Allegedly, an attempt was made in 1968 to poison Alexander Dubček with Sr-90, but it failed.
It is thought that, contrary to popular belief, Gladiators were mostly vegetarian which ensured a greater intake of strontium leading to stronger bones and therefore more resistance to the otherwise bone-breaking and crushing attacks of other gladiators.