Strontium

38 rubidium ← strontium → yttrium Ca ↑ Sr ↓ Ba periodic table
General
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
Physical properties
Phase	solid
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)
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 (Pauling scale)
Ionization energies (more)	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
Miscellaneous
Magnetic ordering	paramagnetic
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
Poisson ratio	0.28
Mohs hardness	1.5
CAS registry number	7440-24-6
Notable 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 89Y β- 0.909D - 90Sr syn 28.90 y β- 0.546 90Y

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. Its properties are closest to those of calcium, and it may replace calcium in bone tissue.

The ⁹⁰Sr isotope is present in radioactive fallout and has a half-life of 28.90 years.

Occurrence and isolation

Strontium occurs commonly in nature amd has been estimated to be the fifteenth most abundant element on Earth, averaging 0.034% 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.

The metallic form of strontium can be prepared by electrolysis of melted strontium chloride mixed with potassium chloride. The reactions at the electrodes can be represented as follows.

cathode: Sr²⁺ + 2 e^- → Sr

anode: 2 Cl^- → Cl_{2 (g)} + 2 e^-

Alternatively, strontium can be produced by reducing strontium oxide with aluminum in a vacuum, at a temperature at which strontium distills off.

History

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.

Strontium was among the radioactive materials released during a fire at a British nuclear reactor at Windscale, Cumbria, in 1957.

Notable characteristics

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.

Volatile strontium salts impart a beautiful crimson color to flames. These salts are used in pyrotechnics and in the production of flares.

Isotopes

In nature, strontium occurs as four stable isotopes: ⁸⁴Sr (0.56%), ⁸⁶Sr (9.86%), ⁸⁷Sr (7.0%), and ⁸⁸Sr (82.58%). 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.

Applications

Commercial uses of strontium and its compounds:

• Strontium is widely used in the screen glass (cathode ray tubes) of color television sets.
• It is commonly used in pyrotechnic displays, for its ability to produce a red color.
• Aerosol paints may contain strontium. This appears to be one of the most likely sources of public exposure to strontium.
• Strontium is used for the production of ferrite magnets and refining zinc.
• Strontium titanate has an extremely high refractive index and optical dispersion, making it useful in a variety of applications in optics. It can act as a diamond simulant, but it is rarely used for that purpose because of its extreme softness and vulnerability to scratching.
• Strontium aluminate is used as a bright phosphor, with long persistence of phosphorescence.
• Strontium chloride is occasionally used in toothpastes for sensitive teeth.
• Strontium oxide is sometimes used to improve the quality of pottery glazes.
• The isotope ⁹⁰Sr is one of the best long-lived, high-energy beta emitters known. It is used as a power source for radioisotope thermoelectric generators (RTGs), which are simple electrical generators that obtain their power from the heat produced by the decay of radioactive elements.[1] These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, and so forth, where a lightweight, long-lived, nuclear-electric power source is required.

Medical uses

• The isotope ⁸⁹Sr is the active ingredient in Metastron, a radioactive pharmaceutical used for treating bone pain secondary to metastatic prostate cancer. The strontium acts like calcium and is preferentially incorporated into bone at sites of increased osteogenesis (bone development). This localization focuses the radiation exposure on the cancerous lesion.

• The radioisotope ⁹⁰Sr is also used in cancer therapy. Its beta emission and long half-life are ideal for superficial radiotherapy.

• An experimental drug made by combining strontium with ranelic acid has aided in bone growth, boosted bone density, and lessened fractures (El-Hajj, 2004; Meunier et al., 2004). Strontium ranelate is registered in Europe for the treatment of osteoporosis.

Precautions

Pure strontium is extremely reactive with air and burns spontaneously. It is therefore considered a fire hazard.

Effects on the human body

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.

See also

• Chemical element
• Periodic table

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Strontium. Retrieved October 6, 2006.
• WebElements.com – Strontium
• El-Hajj Fuleihan G. (2004). Strontium ranelate — a novel therapy for osteoporosis or a permutation of the same?. New England Journal of Medicine 350 (Jan 29): 504-506. PMID 14749460.
• Meunier PJ, Roux C, Seeman E, Ortolani S, Badurski JE, Spector TD, Cannata J, Balogh A, Lemmel EM, Pors-Nielsen S, Rizzoli R, Genant HK, Reginster JY (2004). The effects of strontium ranelate on the risk of vertebral fracture in women with postmenopausal osteoporosis. New England Journal of Medicine 350 (Jan 29): 459-468. PMID 14749454.

External links

• Jefferson Lab: It's Elemental

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