|Name, Symbol, Number||rubidium, Rb, 37|
|Chemical series||alkali metals|
|Group, Period, Block||1, 5, s|
|Atomic mass||85.4678(3) g/mol|
|Electron configuration||[Kr] 5s1|
|Electrons per shell||2, 8, 18, 8, 1|
|Density (near r.t.)||1.532 g/cm³|
|Liquid density at m.p.||1.46 g/cm³|
|Melting point||312.46 K
(39.31 °C, 102.76 °F)
|Boiling point||961 K
(688 °C, 1270 °F)
2093 K, 16 MPa
|Heat of fusion||2.19 kJ/mol|
|Heat of vaporization||75.77 kJ/mol|
|Heat capacity||(25 °C) 31.060 J/(mol·K)|
|Crystal structure||cubic body centered|
(strongly basic oxide)
|Electronegativity||0.82 (Pauling scale)|
|1st: 403.0 kJ/mol|
|2nd: 2633 kJ/mol|
|3rd: 3860 kJ/mol|
|Atomic radius||235 pm|
|Atomic radius (calc.)||265 pm|
|Covalent radius||211 pm|
|Van der Waals radius||244 pm|
|Magnetic ordering||no data|
|Electrical resistivity||(20 °C) 128 nΩ·m|
|Thermal conductivity||(300 K) 58.2 W/(m·K)|
|Speed of sound (thin rod)||(20 °C) 1300 m/s|
|Speed of sound (thin rod)||(r.t.) 2.4 m/s|
|Bulk modulus||2.5 GPa|
|Brinell hardness||0.216 MPa|
|CAS registry number||7440-17-7|
Rubidium (chemical symbol Rb, atomic number 37) is a soft, silvery-white metallic element of the alkali metal group. Rb-87, a naturally occurring isotope, is (slightly) radioactive. Rubidium is very soft and highly reactive, with properties similar to other elements in group one, like rapid oxidation in air.
This element is considered to be the sixteenth most abundant element in the Earth's crust. It occurs naturally in the minerals leucite, pollucite, and zinnwaldite, which contains traces of up to one percent of its oxide. Lepidolite contains 1.5 percent rubidium and this is the commercial source of the element. Some potassium minerals and potassium chlorides also contain the element in commercially significant amounts. One notable source is also in the extensive deposits of pollucite at Bernic Lake, Manitoba.
Rubidium (L rubidus, deepest red) was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff in the mineral lepidolite through the use of a spectroscope. However, this element had minimal industrial use until the 1920s. Historically, the most important use for rubidium has been in research and development, primarily in chemical and electronic applications.
Rubidium is the second most electropositive of the stable alkaline elements and liquefies at high ambient temperature (102.7 F = 39.3 C). Like other group one elements this metal reacts violently in water. In common with potassium and cesium this reaction is usually vigorous enough to ignite the liberated hydrogen. Rubidium has also been reported to ignite spontaneously in air. Also like other alkali metals, it forms amalgams with mercury and it can form alloys with gold, caesium, sodium, and potassium. The element gives a reddish-violet color to a flame, hence its name.
When metallic rubidium reacts with oxygen, as in the tarnishing process, it produces the bronze-colored Rb6O and copper-colored Rb9O2. The final product is principally the superoxide, RbO2, which can then be reduced to Rb2O using excess rubidium metal.
There are 24 isotopes of rubidium known with naturally occurring rubidium being composed of just two isotopes; Rb-85 (72.2 percent) and the radioactive Rb-87 (27.8 percent). Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 to 60 days.
Rb-87 has a half-life of 48.8×109 years. It readily substitutes for potassium in minerals, and is therefore fairly widespread. Rb has been used extensively in dating rocks; Rb-87 decays to stable strontium-87 by emission of a negative beta particle. During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios (ten or higher) occur in pegmatites. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the Sr-87/Sr-86 ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. See Rubidium-Strontium dating for a more detailed discussion.
Potential or current uses of rubidium include:
Rubidium compounds are sometimes used in fireworks to give them a purple color.
Rubidium has also been considered for use in a thermoelectric generator using the magnetohydrodynamic principle, where rubidium ions are formed by heat at high temperature and passed through a magnetic field. These conduct electricity and act like an armature of a generator thereby generating an electric current.
Rubidium, particularly 87Rb, in the form of vapor, is one of the most commonly-used atomic species employed for laser cooling and Bose-Einstein condensation. Its desirable features for this application include the ready availability of inexpensive diode laser light at the relevant wavelength, and the moderate temperatures required to obtain substantial vapor pressures.
Rubidium has been used for polarizing 3He (that is, producing volumes of magnetized 3He gas, with the nuclear spins aligned toward a particular direction in space, rather than randomly). Rubidium vapor is optically pumped by a laser and the polarized Rb polarizes 3He by the hyperfine interaction. Spin-polarized 3He cells are becoming popular for neutron polarization measurements and for producing polarized neutron beams for other purposes.
Rubidium, like sodium and potassium, is almost always in its +1 oxidation state. The human body tends to treat Rb+ ions as if they were potassium ions, and therefore concentrates rubidium in the body's electrolytic fluid. The ions are not particularly toxic, and are relatively quickly removed in the sweat and urine. However, taken in excess it can be dangerous.
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