About

Cesium is a soft, silvery-gold alkali metal that is liquid near room temperature, with a melting point of just 28.5 °C. It is the most reactive and most electropositive of all stable elements, exploding on contact with water. Its most famous use is in cesium fountain atomic clocks, which provide the incredibly precise modern definition of a second. It is also used in drilling fluids.

Atomic Properties

Atomic Number55

Atomic Weight132.905 u

Neutrons (common)78

Electron Configuration[Xe] 6s¹

Full Configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹

Electrons per Shell2, 8, 18, 18, 8, 1

Electronegativity0.79

Electron Affinity45.5 kJ/mol

Oxidation States+1, -1

Covalent Radius225 pm

Van der Waals Radius343 pm

Ionic Radius167 pm

Periodic Table Position

Group1

Period6

BlockS

Chemical SeriesAlkali metals

Physical Properties

State at STPsolid

Appearancesilvery-gold, soft metal, liquid near room temperature

Crystal Structurebcc

Magnetic Orderingparamagnetic

Speed of Sound800 m/s

Poisson Ratio0.29

Thermal Properties

Melting Point301.59 K

Boiling Point944 K

Triple Point301.59 K

Thermal Expansion0 1/K

Thermal Conductivity35.9 W/(m·K)

Specific Heat0.242 J/(g·K)

Molar Heat32.21 J/(mol·K)

Heat of Fusion2.09 kJ/mol

Heat of Vaporization63.9 kJ/mol

Abundance

In Earth's Crust3 ppm

In Ocean0 ppm

In Solar System0 ppm

In Human Body0.021 ppm

Discovery

Discovered byRobert Bunsen, Gustav Kirchhoff

Discovery Year1,860

Occurrenceprimordial

CAS Number7440-46-2

Electron Configuration

Energy

↿⇂

↿

Isotopes of Caesium

Symbol ▲	Mass (u) ▲	Abundance ▲	Half-life ▲	Decay Mode ▲
¹³³Cs	132.905450	100%	Stable	Stable
¹³⁵Cs	134.905970	—	1.33 My	Beta Minus Decay
¹³⁷Cs	136.907080	—	30.04 y	Beta Minus Decay

Interactive Visualization

Explore the 3D Bohr model of Caesium in our interactive viewer.

View 3D Model of Caesium