Cesium (Cs)

Stable Isotopes of Cesium

Isotope Z(p) N(n) Atomic Mass Natural Abundance Nuclear Spin
Cs-133  55  78  132.905447 100.00%   7/2+


Cesium, also spelled caesium, was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff. Its name derives from the Latin word caesius, meaning "sky blue" or "heavenly blue."

Cesium is a golden yellow, soft, ductile metal with a body-centered cubic structure. It is liquid at slightly above room temperature and a soft solid below its melting point. It is highly reactive and decomposes in water, evolving hydrogen, which ignites instantly. It also reacts violently with oxygen, the halogens, sulfur and phosphorus, with spontaneous ignition and/or explosion. Cesium has the highest position in the electromotive series, the lowest melting point of any alkali metal, and the lowest ionization potential of any element. It is soluble in acids and alcohol, reacts explosively with water, and dissolves in liquid ammonia, forming a blue solution. It combines with most nonmetals, forming one or more binary compounds. Combustion with oxygen (or air) first forms the oxide Cs2O, which converts to the peroxide Cs2O2, and then the superoxide CsO2.

Applications of cesium include the following: usefulness as a "getter" in electron tubes, in photoelectric cells, in ion propulsion systems, as heat transfer fluid in power generators, and in atomic clocks. Radioactive Cesium-137 has prospective applications in the sterilization of wheat, flour and potatoes.

Cesium presents a dangerous fire and explosion risk. It ignites spontaneously in moist air, may explode in contact with sulfur or phosphorus, and reacts violently with oxidizing materials. It causes burns on contact with skin.

Properties of Cesium

Name Cesium 
Symbol Cs 
Atomic number 55 
Atomic weight 132.905 
Standard state Solid at 298 ºK (but melts only slightly above this temperature) 
CAS Registry ID 7440-46-2 
Group in periodic table
Group name Alkali metal 
Period in periodic table
Block in periodic table s-block 
Color Silvery gold 
Classification Metallic 
Melting point 28.44 °C
Boiling point 669.3 °C
Thermal conductivity 35.9 W/(m·K) at 298.2 °K
Electrical resistivity 20 µΩ·cm at 20 °C 
Electronegativity 0.7
Specific heat 242 J/kg K
Heat of vaporization 65 kJ·mol-1
Heat of fusion 2.09 kJ·mol-1
Density of liquid 1.843 g/cm3 
Density of solid 1.93 g/cm3 
Mohs hardness scale 0.2
Electron configuration [Xe]6s
Atomic radius 2.65 Å 
Ionic radius Cs+: 1.84 Å
Oxidation state  +1 
Ionization potential 3.89 eV

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