rubidium, Metallic chemical element that is part of the alkali metal group, chemical symbol Rb, atomic number 37. Rubidium is the second most reactive metal after cesium and is very soft, with a silvery white lustre. It was discovered in 1861, and its name derives from the Latin word for red, in reference to rubidium’s prominent red spectral lines. Rubidium is difficult to handle because it ignites spontaneously in air. It is used in photoelectric cells. Rubidium atomic clocks have been constructed.