Rubidium is a silver-white, metallic, active element, which resembles potassium. It ignites in air and reacts with water. It is used in photoelectric cells and electronic vacuum tubes. Rubidium is found world wide in carnallite and pullocite. Symbol Rb; melting point 38.89ºC; atomic weight 85.46; atomic number 37; valence 1, 2, 3, 4.