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Cesium (Cs)

Cesium is a chemical element of the periodic table with chemical symbol Cs and atomic number 55 with an atomic weight of 132.905 u and is classed as alkali metal and is part of group 1 (alkali metal). Cesium is solid at room temperature.

Cesium in the periodic table

Atomic number55
Group1 (Alkali metal)
ClassificationAlkali Metal
AppearanceSilvery gold
Color Silver
Number of protons55 p+
Number of neutrons78 n0
Number of electrons55 e-
From Wikipedia, the free encyclopediaCaesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium.

Physical properties

Phase at STPSolid
Density1.93 g/cm3
Atomic weight132.905 u

Thermal properties

Melting point301.7 K
28.55 °C
83.39 °F
Boiling point944 K
670.85 °C
1239.53 °F
Heat of vaporization67.74 kJ/mol

Atomic properties

Electronegativity (Pauling Scale)0.79
Electron affinity45.505 kJ/mol
Oxidation states−1, +1
(a strongly basic oxide)
Ionization energies
  1. 375.7 kJ/mol
  2. 2234.3 kJ/mol
  3. 3400 kJ/mol

Electron configuration for cesium

Electron configuration
Shorthand configuration
[Xe] 6s1
Electron configuration
Full configuration
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1
Electron configuration chart
Electrons per shell2, 8, 18, 18, 8, 1
Valence electrons 1
Valency electrons 1
Bohr model
CesiumElectron shell for Cesium, created by Injosoft ABCs
Figure: Shell diagram of Cesium (Cs) atom.
Orbital Diagram

The history of Cesium

DiscoveryRobert Bunsen, Gustav Kirchhoff (1860)
First isolationCarl Setterberg (1882)
Discovery of cesium
In 1860, German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Because of the bright blue lines in the emission spectrum, they derived the name from the Latin word caesius, meaning sky-blue. Caesium was the first element to be discovered with a spectroscope, which had been invented by Bunsen and Kirchhoff only a year previously. In 1967, acting on Einstein's proof that the speed of light is the most-constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. Since then, caesium has been widely used in highly accurate atomic clocks.


List of unique identifiers for Cesium in various chemical registry databases
CAS Number7440-46-2
ChemSpider ID4510778
EC number231-155-4
PubChem CID Number5354618