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Caesium facts for kids

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Caesium (or cesium) is the chemical element with the atomic number 55 on the periodic table. Its symbol is Cs.

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silvery gold
Some silvery-gold metal, with a liquid-like texture and lustre, sealed in a glass ampoule
General properties
Name, symbol, number caesium, Cs, 55
Pronunciation SEE-zee-əm
Element category alkali metal
Group, period, block 16, s
Standard atomic weight 132.90545196(6) g/mol
Electron configuration [Xe] 6s1
Electrons per shell 2, 8, 18, 18, 8, 1 (Image)
Physical properties
Density (near r.t.) 1.93 g/cm3
Liquid density at m.p. 1.843 g/cm3
Melting point 301.7 K, 28.5 °C, 83.3 °F
Boiling point 944 K, 671 °C, 1240 °F
Critical point 1938 K, 9.4 MPa
Heat of fusion 2.09 kJ/mol
Heat of vaporization 63.9 kJ/mol
Specific heat capacity (25 °C) 32.210 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 418 469 534 623 750 940
Atomic properties
Oxidation states 1, −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 265 pm
Covalent radius 244±11 pm
Van der Waals radius 343 pm
Crystal structure body-centered cubic
Magnetic ordering paramagnetic
Electrical resistivity (20 °C) 205 nΩ·m
Thermal conductivity (300 K) 35.9 W/(m·K)
Thermal expansion (25 °C) 97 µm/(m·K)
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

Caesium is an alkali metal. Its melting point is low (28 °C). It is extremely reactive. Because of its high reactivity, it is a dangerous chemical. It may set itself on fire (ignite) in air. It explodes on contact with water. It reacts more violently than the other alkali metals with water. Because of this, caesium is stored in mineral oil.

Caesium is a rare element. Since there is little caesium on the Earth, it is rather expensive. The human body does not need caesium. In large amounts, it is mildly poisonous because it is close to potassium, which the body does need.


Caesium was first described in 1861, by Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. They were testing mineral water, from Bad Dürkheim. After they separated calcium, strontium, magnesium and lithium, they saw two lines in the "blue" range of the spectrum. Because of these lines, they concluded that in addition to the elements already found, there must be another unknown substance in the mineral water. They named this substance caesium, after the color blue.

Isotopes and compounds

Caesium has at least 39 known isotopes ranging in atomic mass from 112 to 151. Only one of these, 133Cs, is stable. Therefore, the naturally-occurring isotope of caesium is 133Cs, which is not radioactive. 133Cs is used in atomic clocks, its vibration frequency used to define the length of the second. Another isotope, 137Cs is not made naturally but is made after nuclear fission has been done. It is very radioactive and used as an industrial gamma ray source.

Caesium forms compounds with many other chemical elements. Caesium formate is used in oil drilling because of its high density.

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