Rubidium is a soft, silvery-white metallic element of the alkali metal group. It is highly reactive and shares many properties with potassium and cesium.
Rubidium was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff using flame spectroscopy. Its name comes from the Latin word rubidus, meaning deep red.
Rubidium ignites spontaneously in air and reacts violently with water. It is used in research, specialty glasses, and atomic clocks.