Strontium

Strontium is a chemical element in the periodic table that has the symbol Sr and the atomic number 38. An alkaline earht metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. This metal turns yellow when exposed to air and occurs in celestite and strontianite. Sr-90 is present in radioactive fallout and has a half-life of 28 years.

Properties

General
Name, Symbol, Number	Strontium, Sr, 38
Series	alkaline earth metal
Group, Period, Block	2 (II), 5, s
Density, Hardness	2630 kg/m3, 1.5
Appearance	silvery white metalic
Atomic
Atomic weight	87.62 amu
Atomic radius	219 pm
Covalent radius	192 pm
van der Waals radius	no information
Electron configuration	[Kr]5s2
e- 's per energy level	2, 8, 18, 8, 2
Oxidation states (Oxide)	2 (strong base)
Crystal structure	Cubic face centered
Physical
State of matter	solid (nonmagnetic)
Melting point	1050 K (1431 °F)
Boiling point	1655 K (2520 °F)
Molar volume	33.94 ×10-3 m3/mol
Heat of vaporization	144 kJ/mol
Heat of fusion	8.3 kJ/mol
Vapor pressure	246 Pa at 1042 K
Velocity of sound	data not available
Miscellaneous
Electronegativity	0.95 (Pauling scale)
Specific heat capacity	300 J/kg-K
Electrical conductivity	7.62 106/m ohm
Thermal conductivity	35.3 W/m-K
1st ionization potential	549.5 kJ/mol
2nd ionization potential	1064.2 kJ/mol
3rd ionization potential	4138 kJ/mol
Most Stable Isotopes
iso NA half-life DM DE MeV DP 84Sr 0.56% Strontium is stable with 46 neutrons 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 90Sr {syn.} 28.78 y beta- 0.546 90Y
SI units & STP are used except where noted.

Notable Characteristics
Always naturally occurring combined, as in the minerals strontianite, celestite, etc. It is isolated as a yellowish metal and is somewhat malleable. It is chiefly employed (as in the nitrate) to color pyrotechnic flames red.

Strontium is a bright silvery metal that is softer than calcium and even more reactive in water; strontium will decompose 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. 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.

Uses
The major use for strontium at present is in producing glass for color television cathode ray 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 and so scratches easily. It does not occur naturally.

History
Adair Crawford recognized the mineral strontianite, named after the Scottish town of Strontian, as differing from other barium minerals in 1790. However, Klaproth and Hope discovered strontium itself in 1798 and metallic strontium was first isolated by Sir Humphry Davy in 1808 using electrolysis.

Sources
Strontium is found chiefly as the form of the sulfate mineral celestite (SrSO₄) and the carbonate strontianite (SrCO₃). The metal can be prepared by electrolysis of the melted 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 allotropes of the metal exist, with transition points at 235 and 540°C. Strontium metal (98% pure) in January 1990 cost about $5/oz. The largest commercially exploited deposits are found in England.

Isotopes
The alkali earth metal strontium has four stable, naturally occurring isotopes: ⁸⁴Sr (0.56%), ⁸⁶Sr (9.86%), ⁸¹Sr (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 48,800,000 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. Because strontium has an atomic radius similar to that of Ca, it readily substitutes for Ca in minerals. ⁸⁷Sr/⁸⁶Sr ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0.

Sixteen 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 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 Auxilliary 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 needed.

Precautions

Sources and external links
Los Alamos National Laboratory - Strontium
WebElements.com - Strontium
EnvironmentalChemistry.com - Strontium