Caesium
A golden, ultra-reactive metal defining the SI second.
Inside the Caesium atom
Switch between Bohr and Quantum Cloud modes to compare a simple teaching model with a more realistic probability-based view, and follow the guided tour to explore the Caesium atom step by step.
Electron configuration
[Xe] 6s1
A neutral Caesium atom has 55 electrons (equal to its proton count). Choosing a different isotope above changes only the neutron count.
Shell distribution
Electrons fill inner shells before outer ones; the outermost (valence) shell drives the element's chemistry.
Physical & atomic properties
- State (room temp)
- Solid
- Melting point
- 302 K (28 °C)
- Boiling point
- 944 K (671 °C)
- Density
- 1.873 g/cm³
- Electronegativity
- 0.79 Pauling
- Atomic radius
- 260 pm
- 1st ionization energy
- 376 kJ/mol
- Category
- Alkali metals
Discovery & naming
- Discovered
- 1860
- Discovered by
- Robert Bunsen and Gustav Kirchhoff
- Origin of name
- Latin 'caesius', meaning sky-blue.
Notable uses
Atomic clocks, oil-drilling fluids, and photocells.
Where Caesium comes from
Several comparable sources
Both slow neutron capture in dying stars and rapid capture in explosive events contribute meaningfully.
Simplified origin map — many elements form through more than one astrophysical pathway.
Summary
- Atomic number
- 55
- Atomic mass
- 132.91
- Category
- Alkali metals
- Group · Period
- 1 · 6
- Block
- s-block
- Shells
- 2 · 8 · 18 · 18 · 8 · 1