RUBIDIUM

(Rb), chemical element of Group Ia in the periodic table, the alkali metal group. Rubidium is the second most reactive metal and is very soft with a silvery-white lustre. It was discovered (1861) spectroscopically by Robert Bunsen and Gustav Kirchhoff and named after the two prominent red lines of its spectrum. Rubidium occurs combined in such minerals as lepidolite, pollucite, and carnallite. Small amounts of rubidium are obtained from the mixture of alkali metal carbonates remaining after lithium is extracted from lepidolite. Rubidium ignites spontaneously in air and 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. It is used in photoelectric cells and as a getter in electron tubes to scavenge the traces of sealed-in gases. Rubidium atomic clocks, or frequency standards, have been constructed, but they are not so precise as cesium atomic clocks. Rubidium has been proposed as a working fluid in plasma propulsion for deep-space probes. Natural rubidium makes up about 0.01 percent of the Earth's crust; it exists as a mixture of two isotopes: rubidium-85 (72.15 percent) and the radioactive rubidium-87 (27.85 percent); 15 radioactive isotopes have been artificially prepared, from rubidium-79 to rubidium-95. One estimate of the age of the solar system as 4.6 billion years is based on the ratio of rubidium-87 to strontium-87 in a stony meteorite. Rubidium easily loses its single valence electron and no others, accounting for its electrovalence of one. atomic number 37 atomic weight 85.47 melting point 38.9 C (102 F) boiling point 688 C (1,270 F) specific gravity 1.53 (20 C) valence 1 electronic config. 2-8-18-8-1 or (Kr)5s1