Rubidium

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Rubidium

Rubidium is a chemical element in the periodic table that has the symbol Rb and atomic number 37. Rb is a soft, silvery-white metallic element of the alkali metal group. Rb-87, a naturally occurring isotope, is (slightly) radioactive. Rubidium is highly reactive, with properties similar to other elements in group 1, like igniting spontaneously in air.

Notable characteristics

Rubidium is the second most electropositive of the stable alkaline elements and can be a liquid at room temperature. Like other group 1 elements this metal ignites spontaneously in air and reacts violently in water, liberating and sometimes igniting hydrogen. 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.

Applications

Potential or current uses of Rubidium include:

A working fluid in vapor turbines.
A getter in vacuum tubes.
A photocell component.
The resonant element in atomic clocks. This is due to the hyperfine structure of Rubidium's energy levels.
An ingredient in special types of glass.
The production of superoxide by burning in oxygen.
The study of potassium ion channels in biology.

Rubidium is easily ionized, so it has been considered for use in ion engines for space vehicles (but caesium and xenon are more efficient for this purpose).

Rubidium compounds are sometimes used in fireworks to give them a purple color.

RbAg₄I₅ has the highest room temperature conductivity of any known ionic crystal. This property could be useful in thin film batteries and in other applications.

It 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.

Also, Rubidium is used for polarizing ³He. Polarized Rb polarizes ³He by hyperfine interaction. Polarized ³He cells are becoming popular for polarizing neutrons and also for neutron polarization measurements.

Electrical conductivity 7.79 106/(m·ohm)

History

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.

Occurrence

This element is considered to be the 16th most abundant element in the earth's crust. It occurs naturally in the minerals leucite, pollucite, and zinnwaldite, which contains traces of up to 1% of its oxide. Lepidolite contains 1.5% 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 metal can be produced by reducing rubidium chloride with calcium among other methods. Rubidium forms at least four oxides: Rb₂O, Rb₂O₂, Rb₂O₃, RbO₂. In 1997 the cost of this metal in small quantities was about dollar|US$" target="_blank" >^* 25/gram.

Isotopes

There are 24 isotopes of rubidium known with naturally occurring rubidium being composed of just two isotopes; Rb-85 (72.2%) and the radioactive Rb-87 (27.8%). 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 (10 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.

Rubidium's most common compounds are RbCl, RbF, and Rb₂SO₄.

Precautions

Rubidium reacts violently with water and can cause fires. To ensure both safety and purity, this element must be kept under a dry mineral oil, in a vacuum or in an inert atmosphere.

Biological Effects

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.

References

Los Alamos National Laboratory – Rubidium

External links

Chemical elements | Alkali metals

Louis Meites, Handbook of Analytical Chemistry (New York: McGraw-Hill Book Company, 1963)

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