See Cesium.

Soft, silvery-white, ductile, metallic element, symbol Cs, atomic number 55, atomic weight 132.905. It is one of the alkali metals, and is the most electropositive of all the elements. In air it ignites spontaneously, and it reacts vigorously with water. It is used in the manufacture of photocells.
The rate of vibration of cesium atoms is used as the standard of measuring time. Its radioactive isotope Cs-137 (half-life 30.17 years) is one of the most dangerous waste products of the nuclear industry; it is a highly radioactive biological analog for potassium, produced as a fission product of nuclear explosions and in the reactors of nuclear power plants. It was named in 1860 by Robert Bunsen, German chemist, from the blueness of its spectral line.
A soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal; SYN. caesium, Cs, atomic number 55. + prikaži više

Caesium