ElementsWhat is caesium?
Caesium is a soft, pale gold metal that melts at about 28 degrees Celsius and reacts explosively with water.
What is caesium used for?
Atomic clocks. The second, the world's unit of time, is defined by caesium atoms. Also drilling fluids for oil wells.
Where the name comes from
From the Latin word caesius, sky blue, for the blue lines in its spectrum. It is also spelled cesium.
Key facts
- Symbol
- Cs
- Atomic number
- 55
- Atomic mass
- 132.9054520 u
- Family
- Alkali metal
- Period
- 6
- Group
- 1
- Block
- s
- State at room temperature
- Solid
- Melting point
- 28.4 °C (301.59 K, 83 °F)
- Boiling point
- 670.9 °C (944 K, 1,240 °F)
- Density
- 1.93 g/cm³
- Electrons per shell
- 2, 8, 18, 18, 8, 1
- Electron configuration
- [Xe]6s1
- Common oxidation states
- +1
- Electronegativity (Pauling)
- 0.79
- First ionisation energy
- 3.894 eV
- Electron affinity
- 0.472 eV
- Atomic radius (van der Waals)
- 343 pm
- Discovered
- 1860
Place in the periodic table
Caesium is element number 55. It sits in period 6 (row 6) of the periodic table, in group 1 (column 1), and belongs to the s-block. Just before it comes xenon (54), and just after it comes barium (56). In the same column, rubidium is above it and francium is below it. Elements in one column tend to behave alike.
Family: Alkali metals.
Physical properties
At room temperature caesium is a solid. It melts at 28.4 °C and boils at 670.9 °C, so it stays liquid over a range of 642 degrees. Its density is 1.93 grams per cubic centimetre, so a one-litre block would weigh about 1.93 kilograms. By density it ranks 77th among the 96 elements with a measured value.
Inside the atom
An atom of caesium has 55 protons in its nucleus and, when neutral, 55 electrons around it. The electrons are arranged in 6 shells: 2, 8, 18, 18, 8, 1. The outermost shell holds 1 electron. Written in full, the electron configuration is [Xe]6s1.
Its electronegativity on the Pauling scale is 0.79. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 3.894 electronvolts (about 376 kilojoules per mole). One mole of caesium weighs 132.9054520 grams, so a single gram contains about 4.53 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains caesium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.