38
  Sr  
87.620000
Strontium

Name: Strontium
Symbol: Sr
Atomic Number: 38
Atomic Weight: 87.620000
Family: Alkaline Earth Metals
CAS RN: 7440-24-6
Description: Soft silvery yellow metal that will burn if ignited in air.
State (25C): Solid
Oxidation states: +2

Molar Volume: 33.7 cm3/mole
Valence Electrons: 5s2

Boiling Point:  1657K, 1384C, 2523F
Melting Point:
1042K, 769C, 1416F
Electrons Energy Level: 2, 8, 18, 8, 2
Isotopes: 28 + 4 Stable
Heat of Vaporization: 144 kJ/mol
Heat of Fusion: 8.3 kJ/mol
Density: 2.54 g/cm3 @ 300K
Specific Heat: 0.3 J/gK
Atomic Radius: 2.45
Ionic Radius: 1.12
Electronegativity: 0.95 (Pauling); 0.99 (Allrod Rochow)
Vapor Pressure: 246 Pa @ 769C
4
Be
9.012
Strontium was discovered by Adair Crawford, an Irish chemist, in 1790 while studying the mineral Witherite, BaCO3.  When he mixed Witherite with Hydrochloric Acid, HCl, he did not get the results he expected.  He assumed that his sample of Witherite was contaminated with an unknown mineral, a mineral he named Strontianite SrCO3.

The mineral Strontianite is named after the Scottish village of Strontian having been discovered in the Lead mines there in 1787.  Strontium itself was discovered in 1798 and metallic Strontium was first isolated by Sir Humphry Davy in 1808 using electrolysis.

An alkaline earth metal, Strontium is a soft silver-white or yellowish malleable metallic element that is highly reactive chemically. The metal turns yellow when exposed to air.   Besides Strontianite it also occurs naturally in the mineral Celestine.

It reacts vigorously with water to produce Hydrogen Gas, H2.  It has the same relative abundance as Carbon and Sulfur but does not occur in pure form.

12
Mg
24.30
20
Ca
40.07
38
Sr
87.62
56
Ba
137.3
88
Ra
226.0

1s2 2s2p6 3s2p6d10 4s2p6 5s2

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 compounds are useful in pyrotechnic devices and signal flares because of the bright crimson coloring they give to flames.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6
5s2

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 380C 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.

Occurrence

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- rarrow.gif (63 bytes) Sr

2Cl- rarrow.gif (63 bytes) Cl2 (g) + 2 e-

Alternatively it 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 540C.  The largest commercially exploited deposits are found in England.

Applications

As a pure metal Strontium is being used in Strontium 90%-Aluminum 10% alloys of an eutectic composition for the modification of Aluminum-Silicon casting alloys.  The primary use for Strontium compounds is in glass for color television cathode ray tubes to prevent X-ray emission.

Compounds

Isotopes

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.

Twenty-eight  unstable isotopes are known to exist. Of greatest importance is 90Sr, it has a half-life of about 28.78 years and decays intoYttrium-90 through beta decay.  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.    The strong radiation emitted by the isotope interferes with the production of new blood cells and can cause death.

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

atom.gif (700 bytes)

Isotopes Atomic Mass Half-Life
Sr73 72.966 > 25 ms
Sr74 73.956 > 1.2 ms
Sr75 74.95 71 ms
Sr76 75.942 8.9 seconds
Sr77 76.938 9 seconds
Sr78 77.9322 2.5 minutes
Sr79 78.9297 2.25 minutes
Sr80 79.9245 106.3 minutes
Sr81 80.9232 22.3 minutes
Sr82 81.9184 25.55 days
Sr83 82.9176 32.41 hours
Sr84 83.9134 Stable
Sr85 84.9129 64.84 days
Sr86 85.9093 Stable
Sr87 86.9089 Stable
Sr88 87.9056 Stable
Sr89 88.9075 50.53 days
Sr90 89.9077 28.78 years
Sr91 90.9102 9.63 hours
Sr92 91.911 2.71 hours
Sr93 92.914 7.423 minutes
Sr94 93.9154 75.3 seconds
Sr95 94.9194 23.9 seconds
Sr96 95.9217 1.07 seconds
Sr97 96.9261 429 ms
Sr98 97.9285 0.653 seconds
Sr99 98.933 0.269 seconds
Sr100 99.935 202 ms
Sr101 100.941 118 ms
Sr102 101.943 69 ms
Sr103 102.949 > 150 ns
Sr104 103.952  
Sr105   > 150 ns

Precautions

80px-Flammable.jpg (2186 bytes) In its pure form Strontium is extremely reactive with air and spontaneously combusts.  It is therefore considered to be a fire hazard.

The Strontium unit is used in measuring radioactivity from absorbed 90Sr.

40px-Skull_and_crossbones.svg.jpg (1420 bytes) 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.

An experimental drug made by combining Strontium with Ranelic Acid has aided in bone growth, boosted bone density and lessened fractures.  Women receiving the drug showed a 6.8% increase in bone density.  Women receiving a placebo had a 1.3% 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 is assumed to be a true increase in bone mineral content.  Strontium Ranelate is registered for treatment of osteoporosis in Europe at a dose of 2 grams daily.

Several naturally occurring Strontium compounds have additionally been found to enhance bone growth and density and lessen the incidence of fractures, and efficacy of treatment has not been shown to vary significantly between various Strontium compounds.  These compounds include Strontium Lactate, Strontium Citrate, Strontium Carbonate and Strontium Gluconate.


atom.gif (700 bytes)

Strontium Data

 

Atomic Structure

Atomic Radius (): 2.45
Atomic Volume cm3/mol : 33.7cm3/mol
Covalent Radius: 1.91
Crystal Structure: Cubic face centered
Ionic Radius: 1.12

Chemical Properties

Electrochemical Equivalents: 1.635 g/amp-hr
Electron Work Function: 2.59eV
Electronegativity: 0.95 (Pauling); 0.99 (Allrod Rochow)
Heat of Fusion: 8.3 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 5.695
Second Ionization Potential: 11.03
Third Ionization Potential: 43.6
Valence Electron Potential(-eV): 25.7
Ionization Energy (eV): 5.695 eV

Physical Properties

Atomic Mass Average: 87.62
Boiling Point: 1657K, 1384C, 2523F
Melting Point: 1042K, 769C, 1416F
Heat of Vaporization: 144 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: 23E-6
Electrical Conductivity: 0.0762 106/cm
Thermal Conductivity: 0.353 W/cmK
Density: 2.54 g/cm3 @ 300K
Elastic Modulus (Bulk): 12/GPa
Elastic Modulus (Rigidity): 6.03/GPa
Elastic Modulus Youngs: 15.7/GPa
Enthalpy of Atomization: 163.2 kJ/mole @ 25C
Enthalpy of Fusion: 9.16 kJ/mole
Enthalpy of Vaporization: 150 kJ/mole
Hardness Scale (Brinell): unknown
Hardness Scale (Mohs): 1.5
Hardness Scale (Vickers): unknown
Flammability Class: unknown
Molar Volume: 33.7 cm3/mole
Optical Reflectivity: unknown
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.3 J/gK
Vapor Pressure: 246 Pa @ 769C
Estimated Crustal Abundance: 3.70102 milligrams per kilogram
Estimated Oceanic Abundance:
7.9 milligrams per liter


(Strontian, town in Scotland) Isolated by Davy by electrolysis in 1808; however, Adair Crawford in 1790 recognized a new mineral (strontianite) as differing from other barium minerals. Strontium is found chiefly as celestite and strontianite. The metal can be prepared by electrolysis of the fused 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 allotropic forms of the metal exist, with transition points at 235 and 540oC. Strontium is softer than calcium and decomposes water more vigorously. It does not absorb nitrogen below 380oC. It should be kept under kerosene to prevent oxidation. Freshly cut strontium has a silvery appearance, but rapidly turns a yellowish color with the formation of the oxide. The finely divided metal ignites 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. Sixteen other 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. 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 space vehicles, remote weather stations, navigational buoys, etc., where a lightweight, long-lived, nuclear-electric power source is needed. The major use for strontium at present is in producing glass for color television picture 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. It does not occur naturally. Strontium metal (98% pure) in January 1990 cost about $5/oz.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond


Strontium was discovered by Adair Crawford, an Irish chemist, in 1790 while studying the mineral witherite (BaCO3). When he mixed witherite with hydrochloric acid (HCl) he did not get the results he expected. He assumed that his sample of witherite was contaminated with an unknown mineral, a mineral he named strontianite (SrCO3). Strontium was first isolated by Sir Humphry Davy, an English chemist, in 1808 through the electrolysis of a mixture of strontium chloride (SrCl2) and mercuric oxide (HgO). Today, strontium is obtained from two of its most common ores, celestite (SrSO4) and strontianite (SrCO3), by treating them with hydrochloric acid, forming strontium chloride. The strontium chloride, usually mixed with potassium chloride (KCl), is then melted and electrolyzed, forming strontium and chlorine gas (Cl2).

Most of the strontium produced today is used in the manufacture of color television picture tubes. It is also used to refine zinc and is combined with iron to make magnets.

Two strontium compounds, strontium carbonate (SrCO3) and strontium nitrate (Sr(NO3)2), burn with a bright, red flame and are used in fireworks and signal flares.   Strontium carbonate is also used to make certain kinds of glass and is the base material for making most other strontium compounds.

Strontium-90, a radioactive isotope of strontium, is a common product of nuclear explosions.  It has a half-life of about 28.8 years and decays intoYttrium-90 through beta decay.  Strontium-90 is especially deadly since it has a relatively long half-life, is strongly radioactive and is absorbed by the body, where it accumulates in the skeletal system. The radiation affects the production of new blood cells, which eventually leads to death.