The Online Encyclopedia and Dictionary







Name, Symbol, Number Caesium, Cs, 55
Series Alkali metals
Group, Period, Block 1(IA), 6, s
Density, Hardness 1879 kg/m3, 0.2
Appearance silvery gold
Atomic properties
Atomic weight 132.90545 g/mol
Atomic radius (calc.) 260 (298) pm
Covalent radius 225 pm
van der Waals radius no data
Electron configuration [Xe]6s1
e- s per energy level 2, 8, 18, 18, 8, 1
Oxidation states (Oxide) 1 (strong base)
Crystal structure Cubic body centered
Physical properties
State of matter liquid
Melting point 301.59 K (83.19 °F)
Boiling point 944 K (1240 °F)
Molar volume 70.94 cm3/mol
Heat of vaporization 67.74 kJ/mol
Heat of fusion 2.092 kJ/mol
Vapor pressure 2.5 kPa
Speed of sound no data
Electronegativity 0.79 (Pauling scale)
Specific heat capacity 240 J/(kg·K)
Electrical conductivity 4.89 MS/m
Thermal conductivity 35.9 W/(m·K)
1st ionization potential 375.7 kJ/mol
2nd ionization potential 2234.3 kJ/mol
3rd ionization potential 3400 kJ/mol
Most stable isotopes
iso NA half-life DM DE fJ DP
133Cs 100% Cs is stable with 78 neutrons
134Cs {syn.} 65.159 Ms


135Cs trace 73 Ts
beta- 43.1 135Ba
137Cs {syn.} 948.9 Ms
beta- 188.4 137Ba
SI units & STP are used except where noted.

Caesium (cesium in the USA) pronounced /'siːziəm/; 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 which is one of at least three metals that are liquid at or near room temperature. 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 color and is both soft and ductile. Caesium is also the most electropositive and most alkaline chemical element and also has the least ionization potential of all the elements, except for francium. 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 less than an ounce in the entire earth at one time, its abundance can be considered zero in practical terms.)

Along with gallium 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.


Caesium is most 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 energy levels of the ground state of the caesium-133 atom.

More recently this metal has been used in ion propulsion systems.


Caesium (Latin caesius meaning "sky 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 1881. Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical applications.


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 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 its chloride and its nitrate. The price of caesium in 1997 was about $US 30 per gram.


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 the largest number of isotopes, it has only one naturally occurring stable isotope, Cs-133, the other isotopes (except for the isotopes noted on this page) have half-lives from a few days to factions of a second. The radiogenic isotope Cs-137 has been used in hydrologic studies, analogous to the use of H-3 (tritium). Cs-137 is produced from detonation of nuclear weapons and emissions from nuclear power plants, and notably from the 1986 Chernobyl explosion. Beginning in 1954 with the commencement of nuclear testing, Cs-137 was released into the atmosphere where it is absorbed readily into solution. Once Cs-137 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 is highly explosive in cold water. Caesium should be considered highly toxic. Some of its radioisotopes are even more toxic. Caesium hydroxide is an extremely strong base, and can attack glass.

See also

  • Goiânia accident - a major radioactive contamination incident involving a small rod of caesium chloride


External links

Last updated: 08-16-2005 18:12:37