rubidium (rōbĭdˈēəm) [key], metallic chemical element; symbol Rb; at. no. 37; at. wt. 85.4678; m.p. 38.89°C; b.p. 686°C; sp. gr. 1.53 at 20°C; valence +1. Rubidium is a very soft silver-white metal. One of the alkali metals, it is directly below potassium in Group 1 of the periodic table. It is extremely reactive, combining violently with water to form the hydroxide. It oxidizes rapidly, and may ignite when exposed to air. It forms numerous compounds, e.g., halides, oxides, sulfates, and sulfides. Its salts color a flame red. Rubidium is not found uncombined in nature but occurs widely distributed in lepidolite (the major source), carnallite, pollucite, and some rare minerals, and with lithium in seawater, brines, and natural spring waters. Although rubidium is much more abundant in the earth's crust than chromium, copper, lithium, nickel, or zinc, and about twice as abundant in seawater as lithium, it did not become available commercially until the early 1960s as a byproduct of the manufacture of lithium chemicals. The metal is obtained by electrolysis or chemical reduction of the fused chloride. It must be kept out of contact with air and water. Rubidium and its salts have few commercial uses. The metal is used in the manufacture of photocells and in the removal of residual gases from vacuum tubes. Rubidium salts are used in glasses and ceramics. Rubidium-87, a radioactive isotope (half-life about 5 × 1011 years), makes up about 28% of natural rubidium; the balance is the stable isotope rubidium-85. Fifteen other isotopes are known. Rubidium was discovered with cesium in 1861 by R. W. Bunsen and G. R. Kirchhoff; these were the first elements discovered by spectroscopic analysis.