caesium

Soft, silvery-white, ductile metallic element, atomic number 55, relative atomic mass 132.905. It is one of the alkali metals that form Group 1 of the periodic table of the elements. The alkali metals increase in reactivity down the group, and caesium, with only the short-lived radioactive francium below it, is the most reactive of them all. In air it ignites spontaneously, and it reacts violently with water. It is the most electropositive of all the elements. It is used in the manufacture of photocells.

The rate of vibration of caesium atoms has been used as the standard of measuring time. Its radioactive isotope Cs-137 (half-life 30.17 years) is a product of fission in nuclear explosions and in nuclear reactors; it is one of the most dangerous waste products of the nuclear industry, being a highly radioactive biological analogue of potassium.