Key information
Atomic number: 
55
Atomic weight: 
132.9
Density: 
1900 kg/m³
Boiling point: 
963.16° K
Melting point: 
301.86° K
Heat vapour: 
68.26 kJ/mol
Heat fusion: 
2.09 kJ/mol
Thermal conductivity: 
5.3 W/m/K
Group: 
Alkali Metals

Caesium was discovered in 1860 by Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887) with the aid of a spectroscope they had invented the year previously. It was the first element to be identified through spectrum analysis.

digital clock shows the current timeCaesium-133 is a stable isotope of Caesium widely used in atomic clocks. Time is measured as the transition between two electron spin energy levels. The SI unit, of a second is 9,192,631,770 cycles between transition states of Caesium-133.

Electron Shell: 
Amazon Books
Image of The Last Sorcerers: The Path from Alchemy to the Periodic Table
Author: National Academy of Sciences, Richard Morris
Publisher: Henry (Joseph) Press (2003)
Binding: Hardcover, 294 pages