Symbol Rb. A soft silvery-white metallic element belonging to group 1 (formerly IA) of the periodic table; a.n. 37; r.a.m. 85.47; r.d. 1.53; m.p. 38.89°C; b.p. 688°C. It is found in a number of minerals (e.g. lepidolite) and in certain brines. The metal is obtained by electrolysis of molten rubidium chloride. The naturally occurring isotope rubidium–87 is radioactive (see rubidium-strontium dating). The metal is highly reactive, with properties similar to those of other group 1 elements, igniting spontaneously in air. It was discovered spectroscopically by Robert Bunsen and Gustav Kirchhoff in 1861.
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