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Caesium ( Cs )

Material Information

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General Description:  

Discovered in 1860 by R. Bunsen and G. R. Kirchoff in Heidelberg, Germany.

Caesium is a soft metal which is often liquid at room temperature due to its relatively low melting point (28.5C). It is an extremely reactive metal, reacting violently in contact with water and being rapidly attacked in air. As with other alkaline group metals, caesium can be prepared by electrolysis of the fused halides but, in addition, it can also be prepared by heating the chloride with calcium and distilling out the molten metal. It is not a common element, its abundancy on Earth being 7 ppm.

As a photosensitor, caesium has a peak response at 800nm in the infrared, both thermal- and photo-emission being high. Alloys of caesium with antimony, gallium, indium and thorium are generally photosensitive. Caesium is also used in atomic clocks, a standard measure of time, based on the caesium ion resonance of 9,192,631,770 Hz.

   

Atomic Properties

Atomic number 55
Atomic radius - Goldschmidt ( nm ) 0.27
Atomic weight ( amu ) 133
Crystal structure Body centred cubic
Electronic structure Xe 6s1
Ionisation potential No. eV
1 3.89
2 25.10
Natural isotope distribution Mass No. %
132.91 100
Photo-electric work function ( eV ) 2.14
Thermal neutron absorption cross-section ( Barns ) 29
Valences shown 1

Electrical Properties

Electrical resistivity @20C ( µOhmcm ) 20
Temperature coefficient @0-100C ( K-1 ) 0.0044

Mechanical Properties

Material condition Polycrystalline
Hardness - Mohs 0.2

Physical Properties

Boiling point ( C ) 705
Density @20C ( g cm-3 ) 1.90
Melting point ( C ) 28.5

Thermal Properties

Coefficient of thermal expansion @1-26C ( x10-6 K-1 ) 97
Latent heat of evaporation ( J g-1 ) 611
Latent heat of fusion ( J g-1 ) 15.78
Specific heat @25C ( J K-1 kg-1 ) 230

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