Rubidium
Rubidium, a soft, silvery-white metallic element that is very reactive chemically. When exposed to air, rubidium tarnishes rapidly and may ignite. It will decompose water and, in some cases, ignite the hydrogen freed in the decomposition. For these reasons, pure rubidium is kept under a vacuum or immersed in kerosene or a similar liquid. One use for rubidium is in making photoelectric cells. The metal was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff of Germany.
Chemical symbol: Rb. Atomic weight: 85.4678. Atomic number: 37. Specific gravity: 1.53. Melting point: 102.02F. (38.9C). Boiling point: 1,270.4F. (688C). Valence: 1. Rubidium belongs to Group I-A of the Periodic Table.