Rubidium is a chemical element with symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group, with an atomic mass of 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other alkali metals, such as very rapid oxidation in air. Natural rubidium is a mix of two isotopes: ⁸⁵Rb, the only stable one, constitutes 72% of it. The remaining 28% is the slightly radioactive ⁸⁷Rb with a half-life of 49 billion years—more than three times longer than the estimated age of the universe.