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(ro͞obĭd`ēəm), 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 metalsalkali metals,
metals found in Group 1 of the periodic table. Compared to other metals they are soft and have low melting points and densities. Alkali metals are powerful reducing agents and form univalent compounds.
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, it is directly below potassium in Group 1 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. 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 lithiumlithium
[Gr.,=stone], metallic chemical element; symbol Li; at. no. 3; interval in which at. wt. ranges 6.938–6.997; m.p. about 180.54°C;; b.p. about 1,342°C;; sp. gr. .534 at 20°C;; valence +1. Lithium is a soft, silver-white metal.
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 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. BunsenBunsen, Robert Wilhelm
, 1811–99, German scientist, educated at the Univ. of Göttingen, where he received his doctorate in 1830. He served on the faculties of several universities and was at Heidelberg from 1852 to 1889.
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 and G. R. KirchhoffKirchhoff, Gustav Robert
, 1824–87, German physicist. He served as professor of physics at the universities of Breslau (1850–54), Heidelberg (1854–74), and Berlin (from 1875). He is known especially for his work with the spectroscope in association with R. W.
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; these were the first elements discovered by spectroscopic analysis.



Rb, a chemical element in group I of Mendeleev’s periodic system. Atomic number, 37; atomic mass, 85.4678. A silver-white alkali metal, rubidium occurs in nature as a mixture of the two isotopes—stable 85Rb (72.15 percent) and weakly radioactive 87Rb (half-life T½ = 4.8 × 1010 years). With beta decay, 87Rb forms stable 87Sr. The geological age of rocks and minerals can be reliably determined if the 87Sr and Rb content is known (the rubidium-strontium method). Approximately 20 radioisotopes of rubidium have been obtained artificially.

Rubidium was discovered in 1861 by R. Bunsen and G. Kirchhoff through a spectral analysis of salts that had separated from mineral waters. The element was named for the color of its characteristic red spectral lines (Latin rubidus, red, deep red). Metallic rubidium was first obtained in 1863 by Bunsen.

Distribution in nature. Rubidium is a typical dispersed element. Despite its relatively high concentration in the earth’s crust (clarke)—1.5 × 10-2 percent by weight, greater than the concentration of such metals as Cu, Pb, and Zn—rubidium does not form minerals of its own but rather occurs as an isomorphic admixture in potassium and cesium minerals (sylvite, carnallite, microcline, Rb-muscovite). Rubidium, like potassium, can be found in acidic igneous (granitic) rocks and, especially, in pegmatites (up to 1–3 percent rubidium). Rubidium concentrations in basic and ultrabasic rocks are low, amounting to only 4.5 × 10-3 and 2 × 10-4 percent, respectively. Sea-water contains 1.0 × 10-5 to 2.1 × 10-5 percent rubidium. Rubidium salts are constituents of many mineral springs.

The concentrating minerals lepidolite, zinnwaldite, and pollucite are the richest source of rubidium. Deposits of lithium and potassium minerals containing rubidium are found in, among other countries, the USSR, Czechoslovakia, the German Democratic Republic, Namibia, and Rhodesia. The concentration of rubidium in space amounts to 6.5 atoms of rubidium for every 106 atoms of silicon.

Physical and chemical properties. Rubidium forms soft, silver-white crystals, which display a metallic luster when freshly cut. Its Brinell hardness is 0.2 meganewton/m2 (0.02 kilogram-force/mm2); its crystal form is body-centered cubic, with a = 5.70 angstroms (Å) at 0°C. Rubidium has an atomic radius of 2.48 Å; the ionic radius of Rb+ is 1.49 Å. The element has a density of 1.525 g/cm3 (0°C), a melting point of 38.9°C, a boiling point of 703°C, a specific heat of 335.2 joules/kg/°K (0.08 calorie/g/°C) and a coefficient of linear thermal expansion of 9.0 × 10-5°C-1 (0°-38°C). The modulus of elasticity is 2.4 giganewtons per sq m (240 kilograms-force per sq mm), and the electrical resistivity is 11.29 × 10-6 ohm-cm (20°C). Rubidium is paramagnetic.

The Rb atom readily gives up the lone electron in its outer subshell; the electronic configuration is 5s1. The electronegativity of rubidium is 0.89, and the first ionization potential is 4.176 electron volts. Rubidium is univalent (oxidation number + 1) in all of its compounds. The chemical reactivity of rubidium is extremely high; it reacts vigorously with oxygen to form a peroxide (Rb2O2) and superoxide (RbO2); the oxide Rb2 O is formed if there is insufficient oxygen. Rubidium reacts violently with water, liberating hydrogen and forming a solution of rubidium hydroxide (RbOH). The properties of RbOH bear a strong resemblance to those of potassium hydroxide (KOH). Rubidium combines directly with many nonmetals and reacts vigorously with most acids. Almost all rubidium salts are readily soluble in water. The perchlorate of rubidium (RbClO4), chloroplatinate of rubidium (Rb2[PtCl6]), and certain other salts are only slightly soluble and are used in the analytical determination of Rb together with the methods of flame photometry. Here, the analysis is based on the ability of the vapors of rubidium and rubidium compounds to impart a bright red color to the flame.

Preparation and uses. Rubidium salts are obtained as byproducts in the production of lithium, magnesium, and potassium salts. Metallic rubidium is obtained by the reduction of RbCl with calcium in a vacuum at 700°-800°C. Because of its high reactivity, rubidium must be stored in metal containers under a layer of paraffin oil or in an inert atmosphere inside a glass ampul.

Rubidium is used chiefly as a cathode material for photoelectric cells. It is also added to argon and neon fluorescent tubes to increase the brightness. Rubidium is sometimes added to certain specialized alloys (getters). Rubidium salts are used as catalysts in organic synthesis.


Perel’man, F. M. Rubidii i tsezii, 2nd ed. Moscow, 1960.
Pliushchev, V. E., and B. D. Stepin. Khimiia i tekhnologiia soedinenii litiia, rubidiia i tseziia. Moscow, 1970.


Rubidium in the organism. Rubidium is always present in plant and animal tissues. Terrestrial plants contain approximately 0.00064 percent rubidium; aquatic plants contain only one-half as much. Rubidium accumulates in plants and in the muscles and soft tissues of sea anemones, worms, mollusks, crustaceans, echinoderms, and fishes (build-up factor 8–26). The highest build-up factor (2,600) of the artificial radioisotope86 Rb has been found in the duckweed Lemnapolyrrhiza; among fresh-water invertebrates, the mollusk Galba palustris has the highest factor (370). The ashes of the pectoral muscles of birds have been found to contain 0.0112–0.0135 percent Rb; the ashes of human tissue contain up to 0.01 percent. Human blood contains 0.00032 percent (men) and 0.00028 percent (women). Few studies of the metabolism of rubidium have yet been made.


Borovik-Romanova, T. F. “Rubidii v biosfere.” Trudy biogeokhimicheskoi laboratorii AN SSSR, 1946, vol. 8.
Timofeeva-Resovskaia, E. A. “Raspredelenie radioizotopov po osnovnym komponentam presnovodnykh vodoemov.” Trudy instituta biologii Ural’skogo filiala AN SSSR, 1963, vol. 30.


A chemical element, symbol Rb, atomic number 37, atomic weight 85.4678; a reactive alkali metal; salts of the metal may be used in glass and ceramic manufacture.


a soft highly reactive radioactive element of the alkali metal group; the 16th most abundant element in the earth's crust (310 parts per million), occurring principally in pollucite, carnallite, and lepidolite. It is used in electronic valves, photocells, and special glass. Symbol: Rb; atomic no.: 37; atomic wt.: 85.4678; half-life of 87Rb: 5 × 1011 years; valency: 1, 2, 3, or 4; relative density: 1.532 (solid), 1.475 (liquid); melting pt.: 39.48?C; boiling pt.: 688?C