cesium (Cs)



Cesium (Cs) is a soft, silvery, ductile, metallic element produced as a by-product of zinc refining and by reduction of cesium chloride. It is found mainly as the mineral pollucite. Cesium is one of the alkali metals, found in Group I of the periodic table. It is the most electropositive and alkaline of elements, turns to a liquid at only 28.5°C, and burns spontaneously in moist air. It is used in photoelectric cells, as a catalyst promoter, and to make special glass. The radioisotope cesium-137 can be employed in radiotherapy, but is now rarely used. The cesium (atomic) clock provides the standard measure of time: the electron resonance frequency of the cesium atom is exactly 9,192,631,770 cycles per second. Cesium has also been used in ion propulsion.


Cesium was discovered by Robert Bunsen and Gustav Kirchhoff in Heidelberg, Germany, in 1860. Its name comes from the Latin caesius meaning "sky blue."


atomic number 55
relative atomic mass 132.905
electronic configuration 1s22s22p63s23p64s23d10
first ionization energy 376 kJ/mol
atomic radius 265 pm
ionic radius 167 pm
relative density 1.87
melting point 28.5°C (83.3°F)
boiling point 690°C (1,274°F)