Cesium (Cs)

Soft silvery-white metallic element belonging to group 1 of the periodic table. One of the three metals which are liquid at room temperature. Cs-133 is the natural, and only stable, isotope. Fifteen other radioisotopes exist. Caesium reacts explosively with cold water, and ice at temperatures above 157K. Caesium hydroxide is the strongest base known. Caesium is the most electropositive, most alkaline and has the least ionization potential of all the elements. Known uses include the basis of atomic clocks, catalyst for the hydrogenation of some organic compounds, and in photoelectric cells. Caesium was discovered by Gustav Kirchoff and Robert Bunsen in Germany in 1860 spectroscopically. Its identification was based upon the bright blue lines in its spectrum. The name comes from the latin word caesius, which means sky blue. Caesium should be considered highly toxic. Some of the radioisotopes are even more toxic.
Atomic Number55
Atomic Weight132.90545196
Mass Number133
Protons55 p+
Neutrons78 n0
Electrons55 e-
Cesium.jpg Animated Bohr Model Enhanced Bohr Model Bohr Model Orbital Diagram


Atomic Radius
260 pm
Atomic Volume
Covalent Radius
231.99999999999997 pm
Metallic Radius
235 pm
Ionic Radius
167 pm
Crystal Radius
181 pm
Van der Waals radius
343 pm
1.873 g/cm³
Boiling Point
951.6 K
Melting Point
301.6 K
Electrons per shell2, 8, 18, 18, 8, 1
0.6960954471019 eV/particle
Proton Affinity
Electron Affinity
0.471626 eV/particle
Ionization Potential
3.893905548 eV/particle
Heat of Vaporization
68.3 kJ/mol
Heat of Fusion
2.09 kJ/mol
Heat of Formation
76.5 kJ/mol
Molar Heat Capacity
32.21 J/(mol K)
Specific Heat Capacity
0.242 J/(g⋅K)
Thermal Conductivity
35.9 W/(m K)
Gas Basicity
Dipole Polarizability
400.9 a₀
C6 Dispersion Coefficient
Oxidation States-1, 1
Crystal StructureBody Centered Cubic (BCC)
Lattice Constant
6.05 Å
Bulk Modulus
Electrical Resistivity
Electron Configuration[Xe] 6s1
Magnetic Ordering
Magnetic Susceptibility
Poisson Ratio
Shear Modulus
Young's Modulus
Alternate Names
Adiabatic Index
Electric Conductivity
Critical Pressure
Critical Temperature
Curie Point
Magnetic Susceptibility
Neel Point
Neutron Cross Section
Neutron Mass Absorption
Gas Phase
Quantum Numbers
Refractive Index
Space Group
Speed of Sound
Superconducting Point
Thermal Expansion
Valence Electrons
CategoryAlkali metals, Alkali metals
Glawe Number8
Mendeleev Number5
Pettifor Number8
Geochemical Classalkali metal
Goldschmidt Classlitophile
Decay Mode
Abundance in Earth's crust
Abundance in Oceans
0.0003 mg/L
Abundance in Human Body
Abundance in Meteor
Abundance in Sun
Abundance in Universe8×10-8%

Isotopes of Cesium

Stable Isotopes
Unstable Isotopes
112Cs 113Cs 114Cs 115Cs 116Cs 117Cs 118Cs 119Cs 120Cs 121Cs 122Cs 123Cs 124Cs 125Cs 126Cs 127Cs 128Cs 129Cs 130Cs 131Cs 132Cs 134Cs 135Cs 136Cs 137Cs 138Cs 139Cs 140Cs 141Cs 142Cs 143Cs 144Cs 145Cs 146Cs 147Cs 148Cs 149Cs 150Cs 151Cs


Robert Bunsen and Gustav Kirchhoff were the first to suggest finding cesium in 1860 by spectrum analysis. They discovered cesium by its two blue emission lines in a sample of Dürkheim mineral water. The pure metal was eventually isolated by the German chemist Carl Setterberg while working on his doctorate with Kekulé and Bunsen. From the Latin word caesius, sky blue

DiscoverersGustov Kirchoff, Robert Bunsen
Discovery LocationGermany
Discovery Year1860
Name OriginLatin: coesius (sky blue); for the blue lines of its spectrum.
Cesium compounds are considered to be mildly toxic
Cesium was the first element discovered using a spectroscope


The radioactive isotope cesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Cesium is also used in photoelectric cells and as a catalyst in the hydrogenation of organic compounds. Cesium vapor thermionic generators are low-power devices that convert heat energy to electrical energy. Used as a 'getter' to remove air traces in vacuum and cathode-ray tubes. Also used in producing photoelectric devices and atomic clocks. Since it ionizes readily, it is used as an ion rocket motor propellant.


Found in pollucite [(Cs4Al4Si9O26).H2O] and as trace in lepidolite.