Periodic Table


Atomic number: 55
Atomic weight: 132.90545
Symbol: Cs
Group number: 1
Electronic configuration: [Xe].6s1


Caesium is known as cesium in the USA.

The metal is characterised by a spectrum containing two bright lines in the blue (accounting for its name) along with several others in the red, yellow, and green. It is silvery white, soft, and ductile. It is the most electropositive and most alkaline element. Caesium, gallium, and mercury are the only three metals that are liquid at room temperature. Caesium reacts explosively with cold water, and reacts with ice at temperatures above -116°C. Caesium hydroxide, the strongest base known, attacks glass.

General information

Discoveror: Gustav Kirchhoff, Robert Bunsen
Date discovered: 1860
Discovered at: Germany
Meaning of name: From the Latin word "caesius" meaning "sky blue"

Physical data

Standard state: P


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
133Cs133.905429 (7)100

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Cs.html