Rubidium (Rb), chemical element of Group 1 (Ia) in the periodic table, the alkali metal group. It is the second most reactive metal. Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, silvery-white metal in the alkali metal group. Rubidium metal shares similarities to potassium metal and caesium metal in physical appearance, softness and conductivity
This element was discovered (1861) spectroscopically by German scientists Robert Bunsen and Gustav Kirchhoff and named after the two prominent red lines of its spectrum. Rubidium and cesium often occur together in nature. Rubidium, however, is more widely scattered and seldom forms a natural mineral; it is found only as an impurity in other minerals, ranging in content up to 5 percent in such minerals as lepidolite, pollucite, and carnallite. Brine samples have also been analyzed that contain up to 6 parts per million of rubidium.
It is difficult to handle because it ignites spontaneously in air, and it reacts violently with water to yield a solution of rubidium hydroxide (RbOH) and hydrogen, which bursts into flames; rubidium is therefore kept in dry mineral oil or an atmosphere of hydrogen.
Natural rubidium makes up about 0.01 percent of Earth’s crust; it exists as a mixture of two isotopes: rubidium-85 (72.15 percent) and the radioactive rubidium-87 (27.85 percent), which emits beta rays with a half-life of about 6 × 1011 years. A large number of radioactive isotopes have been artificially prepared, from rubidium-79 to rubidium-95.