Cs
Caesium
Element 55 · Alkali metal
Caesium (Cs)
Caesium is a soft, gold-tinted alkali metal that melts at 28.5 °C and explodes on contact with water.
- Alkali metal
- Period 6
- Group 1
- s-block
- solid at room temperature
Key properties
- Atomic mass
- 132.9055
- u
- Electron configuration
- [Xe] 6s1
- Melting point
- 28.4 °C
- 301.59 K
- Boiling point
- 670.9 °C
- 944 K
- Density
- 1.93 g/cm³
- Electronegativity
- 0.79
- Pauling
- Atomic radius
- 343 pm
- van der Waals
- 1st ionisation energy
- 3.894 eV
Compare with every element:Electronegativity chartAtomic radius chartIonisation energy chartDensity chart of the elementsMelting point chartElectron configuration chart
Did you know?
Notable facts about caesium
- 01
Caesium was the first element discovered by spectroscopy.
- 02
The second is defined by 9,192,631,770 oscillations of a caesium-133 atom.
- 03
It is one of only three metals with a yellowish colour, along with gold and copper.
Applications
What is caesium used for?
- Atomic clocks
- Oil drilling fluids (caesium formate)
- Photoelectric cells
- Catalysts
History
Discovery & name origin
Discovered by
Year
Where
Germany
Name origin
Latin caesius, “sky blue”, for the blue lines in its spectrum.
Atomic structure
Atomic properties
Atomic structure
Atomic properties
- Atomic number
- 55
- Atomic mass
- 132.9055 u
- Group
- 1
- Period
- 6
- Block
- s-block
- Electron configuration
- [Xe] 6s1
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1
- Electrons per shell
- 2, 8, 18, 18, 8, 1
- Electronegativity (Pauling)
- 0.79
- Electron affinity
- 0.472 eV
- Atomic radius (van der Waals)
- 343 pm
- Stable isotopes
- 1
Oxidation states
Highlighted: common oxidation states (+1).
Successive ionisation energies
Energy to remove each electron in turn (kJ/mol). Big jumps show where a new, inner shell begins.
- 1st375.7
- 2nd2,234.3
- 3rd3,400
Physical
Physical properties & colour
Physical
Physical properties & colour
- State at 20 °C
- Solid
- Melting point
- 301.59 K · 28.4 °C · 83.2 °F
- Boiling point
- 944 K · 670.9 °C · 1,239.5 °F
- Density
- 1.93 g/cm³
- Molar heat capacity
- 32.21 J/(mol·K)
- Appearance
- Silvery gold
- CPK model colour
- Copied
The CPK colour is the convention used for this element’s atoms in molecular models.
FAQ
Caesium questions
What is Cs on the periodic table?
Cs is the symbol for caesium, element 55 on the periodic table. It sits in group 1, period 6, in the alkali metal category, and has an atomic mass of 132.9055.
What is the atomic number of caesium?
Caesium has atomic number 55, so every caesium atom has 55 protons in its nucleus. Its symbol is Cs and its atomic mass is 132.9055.
Who discovered caesium?
Caesium was discovered by Robert Bunsen and Gustav Kirchhoff in 1860 (Germany). The name comes from Latin caesius, “sky blue”, for the blue lines in its spectrum.
What is caesium used for?
Caesium is used for: atomic clocks; oil drilling fluids (caesium formate); photoelectric cells; catalysts.
Is caesium a metal?
Yes. Caesium is a alkali metal, one of the metallic elements.
What is the electron configuration of caesium?
The electron configuration of caesium is [Xe] 6s1. Its 55 electrons are arranged in shells as 2, 8, 18, 18, 8, 1.