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Element Fact
Iridium is the most corrosion-resistant metal known. Together with platinum, it is used to make the international prototype of the kilogram, which is the standard definition for the unit of mass.

Cesium, Cs

cesium sample

Atomic number: 55
Atomic mass: 132.90545(2)
Natural abundance: 2.6 ppm
133Cs (132.90545) - 100%

History and Production

Derived from Latin caesius, meaning sky blue due to its characteristic color observed in the spectrum. It was discovered by R.W. Bunsen and G.R. Kirchhoff in 1860 in mineral spa waters by means of spectroscopy. It is obtained commercially as a byproduct in lithium productions. It is also obtained from pollucite mineral (Cs4Al4Si9O26.H2O). The metal can be isolated by means of electrolysis of its fused cyanide. It is used as a getter in electron tubes, photoelectric cells and also as a catalyst in certain hydrogenation processes. It is also used in atomic clocks which are accurate to 5 seconds in 300 years.

Physical Data

It is soft, silvery white metal which can exist as a liquid near the room temperature. It reacts explosively with water. It is distributed (2.6 ppm of crustal rocks) in minerals such as lepidolite, pollucite etc. The metal is used to determined the standard for the time of one second which corresponds to the duration of 9,192,631,770 periods of the energy transition between two hyper-fine levels of the 133Cs at ground state.

Interatomic distance: 531 pm
Melting point: 28.44C
Boiling point: 671C
Thermal conductivity/Wm-1K-1: 35.9 (27C)
Density/kgm-3: 1873 (20C), 1843 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 76.5 kJ/mol
Gibbs free energy of formation: 49.6 kJ/mol
Entropy: 175.6 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Xe] 6s1
Term symbol: 2S1/2
Electron affinity: 45.5050 kJ/mol   Electronegativity (Pauline): 0.79
Ionization energy (first, second, third): 37.704, 2234.36, - kJ/mol

Chemical properties

It is very reactive that ignites spontaneously in air and violently in water to give cesium hydroxide.

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