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37 kryptonrubidiumstrontium


Name, Symbol, Number rubidium, Rb, 37
Chemical series alkali metals
Group, Period, Block 1, 5, s
Appearance grey white
Atomic mass 85.4678(3) g/mol
Electron configuration [Kr] 5s1
Electrons per shell 2, 8, 18, 8, 1
Physical properties
Phase solid
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)
Critical point (extrapolated)
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)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 434 486 552 641 769 958
Atomic properties
Crystal structure cubic body centered
Oxidation states 1
(strongly basic oxide)
Electronegativity 0.82 (Pauling scale)
Ionization energies
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
Mohs hardness 0.3
Brinell hardness 0.216 MPa
CAS registry number 7440-17-7
Notable isotopes
Main article: [[Isotopes of {{{isotopesof}}}]]
iso NA half-life DM DE (MeV) DP
83Rb syn 86.2 d ε - 83Kr
γ 0.52, 0.53,
84Rb syn 32.9 d ε - 84Kr
β+ 1.66, 0.78 84Kr
γ 0.881 -
β- 0.892 84Sr
85Rb 72.168 percent Rb is stable with 48 neutrons
86Rb syn 18.65 d β- 1.775 86Sr
γ 1.0767 -
87Rb 27.835 percent 4.88×1010 y β- 0.283 87Sr

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 metal can be produced by reducing rubidium chloride with calcium, among other methods. Rubidium forms at least four oxides: Rb2O, Rb2O2, Rb2O3, RbO2.


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.

Notable characteristics

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.


  • Rubidium chloride (RbCl): In its gas phase, this salt exists as diatomic molecules,[1] but as a solid it can take one of three arrangements (or polymorphs) as determined with holographic imaging.[2] Solid RbCl is hygroscopic (absorbs moisture from the air), so it is usually protected from atmospheric moisture using a desiccator. It is primarily used in research laboratories. For instance, it is a good electrolyte for electrochemical experiments. For biological and biomedical research, it is used to test the genetic transformation of cells and as a noninvasive biomarker.[3] Infusing tumor cells with rubidium chloride increases their pH.
  • Rubidium hydroxide (RbOH): It is a strong alkali that is formed by dissolving rubidium oxide in water. It is a highly reactive and corrosive compound that burns the skin immediately on contact. It must therefore be handled with extreme care, using protective clothing, gloves, and eye-face protection. It is used mainly in scientific research. Synthesis of nearly all rubidium compounds involves rubidium hydroxide as an intermediate. Rubidium oxide is added to water, and the two react to produce the soluble hydroxide.
  • Rubidium oxide (Rb2O): This yellow colored solid (STP) is the simplest oxide of rubidium. Like other alkali metal oxides, it is a strong base. It thus reacts rapidly with water to form rubidium hydroxide (RbOH), releasing heat. Rubidium oxide is potentially dangerous because, like other strongly alkaline compounds, skin contact can cause burns.


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.

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.

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.


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.

See also


  1. ^ Lide, D.R., P. Cahill, and JL.P. Gold. 1963. Cohesion and polymorphism in solid rubidium chloride Journal of Chemical Physics 40, pp. 156-159. Retrieved January 6, 2008.
  2. ^ Kopecky, M., J. Fábry, J. Kub, E. Busetto, and A. Lausi. 2005. X-ray diffuse scattering holography of a centrosymmetric sample Applied Physics Letters, 87 3p. Retrieved January 6, 2008.
  3. ^ Hougardy, E., P. Pernet, M. Warnau, J. Delisle, and J.C. Grégoire. 2003. Marking bark beetle parasitoids within the host plant with rubidium for dispersal studies Entomologia Experimentalis et Applicata 108, pp. 107. Retrieved January 6, 2008.


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