2007 Schools Wikipedia Selection. Related subjects: Chemical elements

55 xenoncaesiumbarium


Periodic Table - Extended Periodic Table
Name, Symbol, Number caesium, Cs, 55
Chemical series alkali metals
Group, Period, Block 1, 6, s
Appearance silvery gold
Atomic mass 132.9054519 (2) g/mol
Electron configuration [Xe] 6s1
Electrons per shell 2, 8, 18, 18, 8, 1
Physical properties
Phase solid
Density (near r.t.) 1.93 g·cm−3
Liquid density at m.p. 1.843 g·cm−3
Melting point 301.59  K
(28.44 ° C, 83.19 ° F)
Boiling point 944 K
(671 ° C, 1240 ° F)
Critical point 1938 K, 9.4 MPa
Heat of fusion 2.09 kJ·mol−1
Heat of vaporization 63.9 kJ·mol−1
Heat capacity (25 °C) 32.210 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 418 469 534 623 750 940
Atomic properties
Crystal structure body centered cubic
Oxidation states 1
(strongly basic oxide)
Electronegativity 0.79 (Pauling scale)
Ionization energies 1st: 375.7 kJ/mol
2nd: 2234.3 kJ/mol
3rd: 3400 kJ/mol
Atomic radius 260 pm
Atomic radius (calc.) 298 pm
Covalent radius 225 pm
Magnetic ordering no data
Electrical resistivity (20 °C) 205 nΩ·m
Thermal conductivity (300 K) 35.9 W·m−1·K−1
Thermal expansion (25 °C) 97 µm·m−1·K−1
Young's modulus 1.7 GPa
Bulk modulus 1.6 GPa
Mohs hardness 0.2
Brinell hardness 0.14 MPa
CAS registry number 7440-46-2
Selected isotopes
Main article: Isotopes of caesium
iso NA half-life DM DE ( MeV) DP
133Cs 100% Cs is stable with 78 neutrons
134Cs syn 65.159 Ms
(2.0648 y)
ε 1.229 134Xe
β- 2.059 134Ba
135Cs trace 73 Ts
β- 0.269 135Ba
137Cs syn 948.9 Ms
β- 1.176 137Ba

Caesium (also spelled cesium, IPA: /ˈsiːziəm/) is a chemical element in the periodic table that has the symbol Cs and atomic number 55. It is a soft silvery-gold alkali metal with a melting point of 28 °C (83 °F) which makes it one of the metals that are liquid at or near room temperature along with rubidium (39°C), francium (27 °C), mercury (-39 °C), and gallium (30 °C). This element is most notably used in atomic clocks.

The variant spelling cesium is sometimes used, especially in North American English, but caesium is the spelling used by the IUPAC, although since 1993 it has recognized cesium as a variant as well.

Notable characteristics

The electromagnetic spectrum of caesium has two bright lines in the blue part of the spectrum along with several other lines in the red, yellow, and green. This metal is silvery gold in colour and is both soft and ductile. Caesium is also the most electropositive and most alkaline of the stable chemical elements and has the second lowest ionization potential (francium being the lowest). Caesium is the least abundant of the five non-radioactive alkali metals. (Technically, francium is the least common alkali metal, but since it is highly radioactive with an estimated 340 to 550 grams in the entire earth at one time, its abundance can be considered zero in practical terms.)

Along with gallium, francium, and mercury, caesium is among the only metals that are liquid at or near room temperature. Caesium reacts explosively in cold water and also reacts with ice at temperatures above −116°C.

Caesium hydroxide (CsOH) is a very strong base and will rapidly etch the surface of glass. CsOH is often stated to be the "strongest base" (after FrOH), but in fact many compounds such as n-butyllithium and sodium amide are stronger.

There is an account that caesium, reacting with fluorine, takes up more fluorine than it stoichometrically should. It is possible that, after the salt Cs+F has formed, the Cs+ ion, which has the same electronic structure as elemental xenon, can like xenon be oxidised further by fluorine and form traces of a higher fluoride such as CsF3, analogous to XeF2.


Probably the most widespread use of caesium today is in caesium formate-based drilling fluids for the oil industry. The high density of the caesium formate brine (up to 2.3 sg,) coupled with the relative benignity of 133Cs , reduces the requirement for toxic high-density suspended solids in the drilling fluid, which is a significant technological, engineering and environmental advantage.

Caesium is also notably used in atomic clocks, which are accurate to seconds in many thousands of years. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium. SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

  • 134Cs has been used in hydrology as a measure of caesium output by the nuclear power industry. This isotope is used because, while it is less prevalent than either 133Cs or 137Cs, 134Cs can be produced solely by nuclear reactions. 135Cs has also been used in this function.
  • Like other group 1 elements, caesium has a great affinity for oxygen and is used as a " getter" in vacuum tubes.
  • This metal is also used in photoelectric cells due to its ready emission of electrons.
  • Caesium is used as a catalyst in the hydrogenation of certain organic compounds.
  • Radioactive isotopes of caesium are used in the medical field to treat certain types of cancer.
  • Caesium fluoride is widely used in organic chemistry as a base and as a source of anhydrous fluoride ion.
  • Caesium vapor is used in many common magnetometers.
  • Because of their high density, caesium chloride solutions are commonly used in molecular biology for density gradient ultracentrifugation, primarily for the isolation of viral particles, subcellular organelles and fractions, and nucleic acids from biological samples.
  • Cesium nitrate is used as oxidiser to burn silicon in infrared flares like the LUU-19 flare, because it emits much of its light in the near infrared spectrum.
  • More recently this metal has been used in ion propulsion systems.
  • Caesium-137 is an extremely common radioisotope used as a gamma-emitter in industrial applications such as:
    • moisture density gauges
    • leveling gauges
    • thickness gauges
    • well-logging devices (used to measure the thickness of rock-strata)
  • also used as an internal standard in spectrophotometry


Caesium (Latin caesius meaning "sky blue" or "light blue") was spectroscopically discovered by Robert Bunsen and Gustav Kirchhoff in 1860 in mineral water from Dürkheim, Germany. Its identification was based upon the bright blue lines in its spectrum and it was the first element discovered by spectrum analysis. The first caesium metal was produced in 1882 by Carl Setterberg. Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical applications.


Pollucite, a caesium mineral
Pollucite, a caesium mineral

An alkali metal, caesium occurs in lepidolite, pollucite ( hydrated silicate of aluminium and caesium) and within other sources. One of the world's most significant and rich sources of this metal is located at Bernic Lake in Manitoba. The deposits there are estimated to contain 300,000 metric tons of pollucite at an average of 20% caesium.

It can be isolated by electrolysis of fused caesium cyanide and in a number of other ways. Exceptionally pure and gas-free caesium can be made by the thermal decomposition of caesium azide. The primary compounds of caesium are caesium chloride and its nitrate. The price of caesium metal in 1997 was about $US 30 per gram, but its compounds are much cheaper.

See also Caesium minerals.


Caesium has at least 39 known isotopes, which is more than any other element except francium. The atomic masses of these isotopes range from 112 to 151. Even though this element has a large number of isotopes, it has only one naturally occurring stable isotope, 133Cs. Most of the other isotopes have half-lives from a few days to fractions of a second. The radiogenic isotope 137Cs has been used in hydrologic studies, analogous to the use of 3H. 137Cs is produced from the detonation of nuclear weapons and is produced in nuclear power plants, and notably from the 1986 Chernobyl meltdown. Beginning in 1945 with the commencement of nuclear testing, 137Cs was released into the atmosphere where it is absorbed readily into solution and is returned to the surface of the earth as a component of radioactive fallout. Once 137Cs enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes can be estimated as a function of time. Caesium-137 has a half-life of 30.17 years. It decomposes to barium-137m (a short-lived product of decay) then to a form of nonradioactive barium.


All alkaline metals are highly reactive. Caesium, being one of the heavier alkaline metals, is also one of the most reactive and is highly explosive when it comes in contact with water (even cold water, or ice). Caesium hydroxide is an extremely strong base, and can attack glass.

Caesium compounds are encountered rarely by most people. All caesium compounds should be regarded as mildly toxic because of its chemical similarity to potassium. Large amounts cause hyperirritability and spasms, but such amounts would not ordinarily be encountered in natural sources, so Cs is not a major chemical environmental pollutant. Rats fed caesium in place of potassium in their diet die, so this element cannot replace potassium in function.

The isotopes 134Cs and 137Cs (present in the biosphere in small amounts as a result of radiation leaks) represent a radioactivity burden which varies depending on location. Radiocaesium does not accumulate in the body as effectively as many other fission products (such as radioiodine and radiostrontium), which are actively accumulated by the body.

Retrieved from ""