Strontium

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38 rubidiumstrontiumyttrium
Ca

Sr

Ba
Periodic Table - Extended Periodic Table
General
Name, Symbol, Number strontium, Sr, 38
Chemical series 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
Physical properties
Phase solid
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
Heat capacity (25 °C) 26.4  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 796 882 990 1139 1345 1646
Atomic properties
Crystal structure cubic face centered
Oxidation states 2
(strongly basic oxide)
Electronegativity 0.95 (scale Pauling)
Ionization energies
(more)
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
Miscellaneous
Magnetic ordering paramagnetic
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
Poisson ratio 0.28
Mohs hardness 1.5
CAS registry number 7440-24-6
Selected isotopes
Main article: Isotopes of strontium
iso NA half-life DM DE (MeV) DP
82Sr syn 25.36 d ε - 82Rb
83Sr syn 1.35 d ε - 83Rb
β+ 1.23 83Rb
γ 0.76, 0.36 -
84Sr 0.56% Sr is stable with 46 neutrons
85Sr syn 64.84 d ε - 85Rb
γ 0.514D -
86Sr 9.86% Sr is stable with 48 neutrons
87Sr 7.0% Sr is stable with 49 neutrons
88Sr 82.58% Sr is stable with 50 neutrons
89Sr syn 50.52 d ε 1.49 89Rb
β- 0.909D 89Y
90Sr syn 28.90 y β- 0.546 90Y
References

Strontium (IPA: /ˈstrɒntiəm/) is a chemical element in the periodic table that has 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.

Contents

  • 1 Notable characteristics
  • 2 Applications
  • 3 Compounds
  • 4 History
  • 5 Occurrence
  • 6 Isotopes
  • 7 Precautions
  • 8 Effect on the human body
  • 9 See also
  • 10 References

[edit] Notable characteristics

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, which strontium decomposes on contact with 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 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.

[edit] Applications

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 color television cathode ray tubes to prevent X-ray emission.

Other uses:

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 the deep 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 system to which Sr isotope ratio studies have been frequently employed is the River Nile (Krom et al, 1999; Krom et al, 2002; Talbot et al. 2000). Due to the vastly 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 beyond, can be discerned through Sr isotopic studies. This information is useful as it can elicit information regarding palaeoclimate change.

[edit] Compounds

[edit] History

The mineral strontianite is named after the Scottish village of Strontian, having been discovered in the lead mines there in 1787.[2] 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.

Strontium was among the radioactive materials released by the 1957 Windscale fire.

[edit] Occurrence

Strontium output in 2005

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.

See also strontium minerals.

[edit] Isotopes

Main article: Isotopes of strontium

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 is 90Sr with a half-life of 28.78 years. It 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.

[edit] Precautions

In its pure form strontium is extremely reactive with air and spontaneously combusts. It is therefore considered to be a fire hazard.

[edit] 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 do 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.

An innovative drug made by combining strontium with ranelic acid has aided in bone growth, boosted bone density and lessened vetrebral, peripheral and hip fractures.[3] [4] 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 and strong 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 presentations given the IOF World Congress on Osteoporosis, in June of 2006. It also reduced bone resorbtion.

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 a 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.

An attempt was made in 1968 to poison Alexander Dubček with Sr-90, but it failed.