Rubidium, << roo BIHD ee uhm, >> (chemical symbol, Rb) is a soft, silvery-white metallic element. It is used in the making of photoelectric cells and vacuum tubes and as a catalyst (substance that stimulates a chemical reaction). Pure rubidium does not occur in nature because it is extremely reactive. It burns on exposure to air and reacts violently with water and acids. Rubidium compounds occur widely in Earth’s crust. Rubidium’s atomic number (number of protons in its nucleus) is 37. Its relative atomic mass is 85.4678. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. Rubidium melts at 39.31 °C and boils at 688 °C. The German scientists Gustav Kirchhoff and Robert Bunsen discovered the element in 1861.
Chemists classify rubidium as an alkali metal . For information on the position of rubidium on the periodic table, see the article Periodic table .
