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Caesium,  55Cs
General properties
Name, symbol Caesium, Cs
Appearance Pale gold
Caesium in the periodic table


Atomic number 55
Standard atomic weight (Ar) 132.90545196(6)
Group, block (alkali metals); s-block
Period period 6
Electron configuration [Xe] 6s1
per shell
2, 8, 18, 18, 8, 1
Physical properties
Pale gold
Phase Solid
Melting point 301.7 K ​(28.5 °C, ​83.3 °F)
Boiling point 944 K ​(671 °C, ​​1240 °F)
Density near r.t. 1.93 g/cm3
when liquid, at  1.843 g/cm3
Critical point 1938 K, 9.4 MPa
Heat of fusion 2.09 kJ/mol
Heat of 63.9 kJ/mol
Molar heat capacity 32.21 J/(mol·K)
Atomic properties
Oxidation states +1, −1​ ​(a strongly basic oxide)
Electronegativity Pauling scale: 0.79
energies 1st: 375.7 kJ/mol
2nd: 2234.3 kJ/mol
3rd: 3400 kJ/mol
Atomic radius empirical: 265 pm
Covalent radius 244±11 pm
Van der Waals radius 343 pm
Crystal structure ​Body-centred cubic (bcc)
Thermal expansion 97 µm/(m·K) (at 25 °C)
Thermal conductivity 35.9 W/(m·K)
Electrical resistivity 205·10-9 Ω·m (at 20 °C)
Magnetic ordering Paramagnetic
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
Naming From Latin caesius - sky blue, for its spectral colours
Discovery Robert Bunsen and Gustav Kirchhoff (1860)
First isolation Carl Setterberg (1882)
· references

Caesium or cesium is an alkali metal with the chemical symbol Cs and atomic number 55. It is the heaviest, stable alkali metal.



Caesium is an extremely reactive metal an will spontaneously ignite in air to form caesium oxides and hydroxides. The reaction with water is explosive, capable of shattering the water's glass container.

Cs + H2O → CsOH + ½ H2

Caesium also forms two unusual acid nitrates, CsNO3·HNO3 and CsNO3·2HNO3

Caesium hydroxide is extremely corrosive and attack glass much faster than the other alkali hydroxides.


Caesium is a soft alkali metal, silvery-gold in color. It has a very low melting point of 28 °C, meaning it can be a liquid at near room temperature or if held in one's hand. It is the least electronegative element and is extremely reactive and even pyrophoric. It will react with water even at temperatures as low as −116 °C.


Cesium ampoules can be purchased from GalliumSource, however it is extremely expensive, a 100 g ampoule is 2400 $ and a small 1 g one is 140 $. Another seller, SmartElements sells cesium ampoules cheaper, a 10 g ampoule is 179 € while a 1 g ampoule is only 79 €.

Very small amounts of cesium alloys are used in the cathode of the electron gun from the cathode ray tube TVs.[1] The exact composition of the alloy varies, depending on the generation of tube, as not all of them have cesium.[2]

Cesium compounds, such as cesium formate, are used as drilling fluids.


Because it's extremely reactive, isolating pure caesium metal is extremely difficult. One way is to reduce caesium halides with a reactive metal such as calcium, barium, or lithium at 700-800 °C, followed by distillation of the caesium metal. YouTube vlogger thunderf00t has performed this preparation, as it seems to be much cheaper to perform this extraction than it is to buy the metal (but it is far more convenient to just buy the metal).

Purer caesium metal can also be prepared by decomposing caesium azide in vacuum at 390 °C, which is prepared from caesium sulfate and barium azide.

Electrolytic preparation of elemental cesium is extremely difficult. Caesium chloride for example, melts at at 645 °C, while cesium metal boils at 671 °C, so without a sensitive temperature controller there's a risk of boiling the metal. This boiling, however, tends to drive isolation reactions forward by increasing entropy.




Caesium metal is extremely reactive and pyrophoric in air. Cesium will not usually catch fire just by being exposed to air, but friction, heating, or exposure to water can trigger a fire or explosion.

In its ionic form, caesium ions are extremely similar to potassium and rubidium ions, and its toxicity is just as low as the mentioned elements.


While it can be stored under mineral oil, it will oxidize much faster than lithium, sodium or potassium, so it is best stored in vacuum containers or argon ampoules. The best way to handle metallic cesium is in a glove box filled with inert gas, such as argon.


Cesium metal cannot be safely neutralized by dissolving it in isopropanol, like sodium, as the reaction is just as violent as the one with water. It is best to leave it in air (or in a CO2 atmosphere, as there's a less risk of fire) to turn into oxide, hydroxide and carbonate, that can be safely disposed of.[3] Due to its rarity, it's best to recycle caesium.


  3. Safety in the Chemistry and Biochemistry Laboratory By André Picot, P. Grenouillet, p. 213

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