Name: Cesium
Symbol: Cs
Atomic Number: 55
Atomic Weight: 132.905450
Family: Alkali Metals
CAS RN: 7440-46-2
Description: A silvery white, soft, and ductile alkali metal.
State (25C): Solid
Oxidation states: +1

Molar Volume: 70.73 cm3/mole
Valence Electrons: 6s1

Boiling Point:  944K, 671C, 1240F
Melting Point:
301.7K, 28.55C, 83.39F
Electrons Energy Level: 2, 8, 18, 18, 8, 1
Isotopes: 39 + 1 Stable
Heat of Vaporization: 67.74 kJ/mol
Heat of Fusion: 2.092 kJ/mol
Density: 1.873 g/cm3 @ 300K
Specific Heat: 0.24 J/gK
Atomic Radius: 3.34
Ionic Radius: 1.67
Electronegativity: 0.79 (Pauling); 0.86 (Allrod Rochow)
Caesium (Latin, caesius meaning "sky blue" or "light blue") was spectroscopically discovered by German chemists Robert William Bunsen and Gustav Robert Kirchhoffin 1860 in mineral water from Drkheim, 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 Cesium metal was produced in 1882 by Carl Setterberg.  Historically, the most important use for Cesium has been in research and development, primarily in chemical and electrical applications.

Also spelled Cesium, 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 (39C), 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.

Cesium is so reactive that it will even explode on contact with ice.  It has been used as a "getter" in the manufacture of vacuum tubes (i.e., it helps remove trace quantities of remaining gases). An isotope of Cesium is used in the atomic clocks.


1s2 2s2p6 3s2p6d10 4s2p6d10 5s2p6 6s1


The electromagnetic spectrum of Cesium 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.  Cesium is the second most electropositive and alkaline of the chemical elements and has the second lowest ionization potential (after Francium).  Cesium 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 550 grams in the entire Earth's crust at one time,  its abundance can be considered zero in practical terms.)

Cesium has the second lowest melting point of all metallic elements, which limits its uses. Cesium readily combines with Oxygen and is used as a getter, a material that combines with and removes trace gases from vacuum tubes. Cesium is also used in atomic clocks, in photoelectric cells and as a catalyst in the hydrogenation of certain organic compounds. Since it is easily ionized and has a high mass, cesium ions may one day be used as a propellant in ion engines on spacecraft.

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10
5s2 5p6

Along with Gallium, Francium, and Mercury, Cesium is among the only metals that are liquid at or near room temperature.  Cesium reacts explosively in cold water and also reacts with ice at temperatures above -116C (157K).

Cesium 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 Cesium, reacting with Fluorine, takes up more Fluorine than it stoichiometrically 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 oxidized further by Fluorine and form traces of a higher Fluoride such as CsF3, analogous to XeF2.


Today, cesium is primarily obtained from the mineral Pollucite (CsAlSi2O6).  Obtaining pure Cesium is difficult since Cesium ores are frequently contaminated with Rubidium, an element that is chemically similar to Cesium.  To obtain pure Cesium, Cesium and Rubidium ores are crushed and heated with Sodium metal to 650C, forming an alloy that can then be separated with a process known as fractional distillation.  Metallic Cesium is too reactive to easily handle and is usually sold in the form of Cesium Azide (CsN3).  Cesium is recovered from Cesium Azide by heating it.

An alkali metal, Cesium occurs in Lepidolite, Pollucite (hydrated Silicate of Aluminum and Cesium) and within other sources. One of the world's most significant and rich sources of this metal is at Bernic Lake in Manitoba, Canada.  The deposits there are estimated to contain 300,000 metric tons of Pollucite at an average of 20% Cesium.

It can be isolated by electrolysis of fused Cesium Cyanide and in a number of other ways.  Exceptionally pure and gas-free Cesium can be made by the thermal decomposition of Cesium Azide.  The primary compounds of Cesium are Cesium Chloride and its Nitrate.  The price of Cesium metal in 1997 was about $30 per gram, but its compounds are much cheaper.


Probably the most widespread use of Cesium today is in Cesium Formate-based drilling fluids for the oil industry.  The high density of the Cesium 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.

Cesium 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 Cesium.  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.


Cesium Chloride (CsCl) and Cesium Nitrate (CsNO3) are Cesium's most common compounds and are primarily used in the production of other chemicals.

Cesium Hydroxide, CsOH Cesium Cyanide, CsCN
Cesium Azide, CsN3 Cesium Fluoride, CsFl
Cesium Chloride, CsCl Cesium Nitrate, CsNO3
Pollucite, CsAlSi2O6


Cesium 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 was released to the atmosphere most notably from the 1986 Chernobyl meltdown.   It's because this isotope (137Cs) is one of the numerous products of fission, which directly issue from the fission of an Uranium core.

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.  Cesium-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.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
Cs112 111.95 500 us
Cs113 112.945 17 us
Cs114 113.941 0.57 seconds
Cs115 114.936 1.4 seconds
Cs116 115.933 3.84 seconds
Cs117 116.9286 8.4 seconds
Cs118 117.9266 14 seconds
Cs119 118.9224 43 seconds
Cs120 119.9207 64 seconds
Cs121 120.9172 155 seconds
Cs122 121.9161 21 seconds
Cs123 122.913 5.94 minutes
Cs124 123.9122 30.8 seconds
Cs125 124.9097 45 minutes
Cs126 125.9094 1.64 minutes
Cs127 126.9074 6.25 hours
Cs128 127.9078 3.66 minutes
Cs129 128.9061 32.06 hours
Cs130 129.9067 29.21 minutes
Cs131 130.9055 9.689 days
Cs132 131.9064 6.479 days
Cs133 132.9055 Stable
Cs134 133.9067 2.0648 years
Cs135 134.906 2300000 years
Cs136 135.9073 13.16 days
Cs137 136.9071 30.07 years
Cs138 137.911 33.41 minutes
Cs139 138.9134 9.27 minutes
Cs140 139.9173 63.7 seconds
Cs141 140.92 24.94 seconds
Cs142 141.9243 1.7 seconds
Cs143 142.9273 1.78 seconds
Cs144 143.932 1.01 seconds
Cs145 144.9354 0.594 seconds
Cs146 145.9402 0.321 seconds
Cs147 146.944 0.225 seconds
Cs148 147.949 158 ms
Cs149 148.953 >50 ms
Cs150 149.958 >50 ms
Cs151 150.962 > 50 ms


80px-Flammable.jpg (2186 bytes) All alkali metals are highly reactive. Cesium, being one of the heavier alkali metals, is also one of the most reactive and is highly explosive when it comes in contact with water (even cold water or ice).  Cesium reacts violently with water and ice, forming cesium hydroxide (CsOH). Cesium hydroxide is the strongest base known and will attack glass.

  Cesium Hydroxide is an extremely strong base, and can etch glass.

40px-Skull_and_crossbones.svg.jpg (1420 bytes) Cesium compounds are encountered rarely by most persons . All Cesium 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 Cesium 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.  Radiocesium 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.

atom.gif (700 bytes)

Cesium Data


Atomic Structure

Atomic Radius (): 3.34
Atomic Volume cm3/mol : 71.07cm3/mol
Covalent Radius: 2.35
Crystal Structure: Cubic body centered
Ionic Radius: 1.67

Chemical Properties

Electrochemical Equivalents: 4.9587 g/amp-hr
Electron Work Function: 2.14eV
Electronegativity: 0.79 (Pauling); 0.86 (Allrod Rochow)
Heat of Fusion: 2.092 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 3.894
Second Ionization Potential: 25.1
Third Ionization Potential: unknown
Valence Electron Potential(-eV): 8.62
Ionization Energy (eV): 3.894 eV

Physical Properties

Atomic Mass Average: 132.9054
Boiling Point: 944K, 671C, 1240F
Melting Point: 301.7K, 28.55C, 83.39F
Heat of Vaporization: 67.74 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: 97E-6
Electrical Conductivity: 0.0489 106/cm
Thermal Conductivity: 0.359 W/cmK
Density: 1.873 g/cm3 @ 300K
Elastic Modulus (Bulk): 1.6/GPa
Elastic Modulus (Rigidity): 0.65/GPa
Elastic Modulus Youngs: 1.7/GPa
Enthalpy of Atomization: 78.2 kJ/mole @ 25C
Enthalpy of Fusion: 2.1 kJ/mole
Enthalpy of Vaporization: 65.9 kJ/mole
Hardness Scale (Brinell): 0.14 MN m-2
Hardness Scale (Mohs): 0.2
Hardness Scale (Vickers): unknown
Flammability Class: unknown
Molar Volume: 70.73 cm3/mole
Optical Reflectivity: unknown
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.24 J/gK
Vapor Pressure: 2.5 kPa
Estimated Crustal Abundance: 3 milligrams per kilogram
Estimated Oceanic Abundance:
310-4 milligrams per liter

(L. caesius, sky blue) Cesium was discovered spectroscopically by Bunsen and Kirchhoff in 1860 in mineral water from Durkheim. Cersium, an alkali metal, occurs in lepidolite, pollucte (a hydrated silicate of aluminum and cesium), and in other sources. One of the world's richest sources of cesium is located at Bernic Lake, Manitoba. The deposits are estimated to contain 300,000 tons of pollucite, averaging 20% cesium. It can be isolated by electrolysis of the fused cyanide and by a number of other methods. Very pure, gas-free cesium can be prepared by thermal decomposition of cesium azide. The metal is characterized by a spectrum containing two bright lines in the blue along with several others in the red, yellow, and green. It is silvery white, soft, and ductile. It is the most electropositive and most alkaline element. Cesium, gallium, and mercury are the only three metals that are liquid at room temperature. Cesium reacts explosively with cold water, and reacts with ice at temperatures above -116oC. Cesium hydroxide, the strongest base known, attacks glass. Because of its great affinity for oxygen the metal is used as a "getter" in electron tubes. It is also used in photoelectric cells, as well as a catalyst in the hydrogenation of certain organic compounds. The metal has recently found application in ion propulsion systems. Cesium is used in atomic clocks, which are accurate to 5 s in 300 years. Its chief compounds are the chloride and the nitrate. Cesium has 32 isotopes (more than any element) with masses ranging from 114 to 145. The present price of cesium is about $30/g.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond