strontium

38 rubidium ← strontium → yttrium Ca ↑ Sr ↓ Ba Periodic Table - Extended Periodic Table
General
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
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
Specific 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, 1[1] (strongly basic oxide)
Electronegativity	0.95 (Pauling scale)
Ionization energies (more)	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
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
Most-stable 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% 84Sr is stable with 46 neutrons 85Sr syn 64.84 d ε - 85Rb γ 0.514D - 86Sr 9.86% 86Sr is stable with 48 neutrons 87Sr 7.0% 87Sr is stable with 49 neutrons 88Sr 82.58% 88Sr 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 (pronounced /ˈstrɒnʃiəm/, /ˈstrɒntiəm/, or /ˈstrɒnʃəm/) 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.

Characteristics

Dendritic oxidized strontium

Due to its extreme reactivity with oxygen and water, this element occurs naturally only in compounds with other elements, as in the minerals strontianite and celestite.

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 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.

Compounds

• 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 color.
• 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 ranelate is used in the treatment of osteoporosis. It is a prescription drug in the EU, but not in the USA.

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 during primordial nucleosynthesis 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 is used in the treatment of bone cancer. 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.

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 of a mixture containing strontium chloride and mercuric oxide and announced by him in a lecture to the Royal Society on 30 June 1808^[3].

Occurrence

Strontium output in 2005

According to the British Geological Survey, China was the top producer of strontium in 2007, with over two-thirds world share, followed by Spain and Mexico.^[4]

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.^[5] 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.

Applications

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.^[6] The primary use for strontium compounds is in glass for colour television cathode ray tubes to prevent X-ray emission.^[7][8]

Other uses:

• ⁸⁹Sr is the active ingredient in Metastron, a radiopharmaceutical used for bone pain secondary to metastatic bone 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).^[9] 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.

• Strontium is used in studies of neurotransmitter release in neurons. Like calcium, strontium facilitates synaptic vesicle fusion with the synaptic membrane. But unlike calcium, strontium causes asynchronous vesicle fusion. Therefore, replacing calcium in the culture medium with strontium allows scientists to measure the effects of a single vesicle fusion event, e.g., the size of the postsynaptic response elicited by the neurotransmitter content of a single vesicle.^[10][11]

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.^[12] A good example of a fluvial-marine system to which Sr isotope provenance studies have been successfully employed is the River Nile-Mediterranean system ^{[13] [14][15]}. Due to the differing ages of the rocks that constitute the majority of the Blue and White Nile catchment areas of 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^{[16] [17]}. ⁸⁷Sr/⁸⁶Sr ratios in teeth may also be used to track animal migrations ^[18][19] or in criminal forensics.

Strontium atoms are used in an experimental atomic clock with record-setting accuracy.^[20]

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.^[21]

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.^[22][23] 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.

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. Their long-term safety and efficacy have never been evaluated on humans using large-scale medical trials.

References