Caesium was discovered, in 1860, by German scientists Robert Bunsen and Gustav Kirchhoff. They burned a sample of mineral water in a burner, which split the light from the flame into individual colors. One of them was a peculiar light blue, which came from cesium.

This metal is the most reactive metal on Earth, and explodes in flames on contact with air or water. Therefore, pure cesium is stored in vacuum-sealed glass tubes. This element is rare, and most of it is extracted from the mineral polucite. Its name means "sky blue" and refers to the color of the cesium flame as it burns. It is used in atomic clocks, which measure time to an accuracy of up to one billionth of a second (± one second error every 300 years).

Summary of properties (Cs)

Atomic weight132.90545196(6)
Discoverer (year)Kirchhoff, Gustav & Bunsen, Robert (1860)
Natural formmetallic solid (body centered cubic)
Electron configuration[Xe] 6s1
Melting point (ºC)29
Boiling point (ºC)678
Abundance in earth's crust (ppm)3
Isotope (abundance)133Cs (100)
Density g/cm31.87
Van der Waals radius (pm)343
Covalent radius (pm)238
Electronegativity (Pauling)0.79
Vaporization enthalpy (kJ/mol)65.90
Enthalpy of fusion (kJ/mol)2.09
Specific heat capacity (J/g·K) at 25ºC and 1 atm0.24
Thermal conductivity (W/cm·K) at 25ºC and 1 atm0.360
Oxidation state+1
Electron affinity (eV)0.47
1st Ionization potential (eV)3.8939

Back to the Periodic Table of the Elements.