What is caesium used for?

Caesium's primary modern use is in atomic clocks — the SI second is defined by the hyperfine transition frequency of caesium-133. It is also used in photoelectric cells, ion propulsion engines, and as a catalyst in certain chemical reactions. Caesium formate is used as a high-density drilling fluid in the oil industry.

Is caesium dangerous?

Elemental caesium is highly reactive and must be stored under an inert atmosphere. It ignites spontaneously in air and reacts explosively with water. Caesium-137, a radioactive isotope, is a significant radiation hazard produced in nuclear fission. Stable caesium-133 compounds are generally of low toxicity but should still be handled with standard laboratory precautions.

Why does caesium have such a low melting point?

Caesium's very large atomic radius (298 pm) means its outer valence electron is loosely held and its metallic bonds are weak. The combination of low charge density and large atomic size results in a melting point of only 28.5 °C — well below body temperature — making it liquid in a warm room.

What is the difference between caesium and cesium?

They are the same element. "Caesium" is the IUPAC-recommended and British spelling; "cesium" is the common American spelling. Both refer to element 55 with symbol Cs.

How is caesium-133 used to define the second?

The SI second is defined as exactly 9,192,631,770 oscillations of the electromagnetic radiation corresponding to the hyperfine transition of caesium-133 atoms in the ground state. Caesium atomic clocks are accurate to about 1 second in 300 million years.

What group and period is caesium in?

Caesium is in Group 1 (alkali metals) and Period 6 of the periodic table. It sits below rubidium and above francium in the same group.

Caesium Element Properties

Explore the full physical and chemical data for caesium (Cs), element 55 — from atomic mass and electron configuration to melting point and real-world applications.

55 Cs 132.905

Caesium

Alkali Metal · Period 6 · Group 1 · Block s

A soft, silvery-gold alkali metal that is liquid near room temperature and one of the most reactive elements known.

Solid at STP Highly Reactive Atomic Clocks

Element Name	Caesium (Cesium)
Symbol	Cs
Atomic Number	55
Atomic Mass	132.905 u
Category	Alkali Metal
Group	1 (IA)

Period	6
Block	s-block
Standard State	Solid at 25 °C
Appearance	Silvery-gold, soft metal
Discovery	Robert Bunsen & Gustav Kirchhoff, 1860
Name Origin	Latin caesius — "sky blue" (spectral lines)

Property	Value
Atomic Number (Z)	55
Mass Number (most stable)	133
Atomic Mass	132.90545196 u
Electron Configuration	[Xe] 6s¹
Electrons per Shell	2, 8, 18, 18, 8, 1
Valence Electrons	1
Oxidation States	+1 (dominant), −1 (rare)
Electronegativity (Pauling)	0.79 (lowest of stable elements)
Electron Affinity	45.51 kJ/mol
Ionization Energy (1st)	375.7 kJ/mol
Ionization Energy (2nd)	2234.3 kJ/mol
Atomic Radius (calculated)	298 pm
Covalent Radius	244 pm
Van der Waals Radius	343 pm
Nuclear Spin (Cs-133)	7/2
Nuclear Magnetic Moment	+2.5820 μN
Hyperfine Transition (Cs-133)	9,192,631,770 Hz (defines the SI second)
Neutrons (Cs-133)	78
Protons	55

Property	Value
Melting Point	28.5 °C (301.65 K, 83.3 °F)
Boiling Point	671 °C (944.15 K, 1240 °F)
Density (at 20 °C)	1.873 g/cm³
Phase at STP	Solid
Crystal Structure	Body-centered cubic (BCC)
Lattice Constant	6.141 Å
Heat of Fusion	2.09 kJ/mol
Heat of Vaporization	63.9 kJ/mol
Specific Heat Capacity	0.242 J/(g·K)
Thermal Conductivity	35.9 W/(m·K)
Thermal Expansion	97 µm/(m·K) at 25 °C
Electrical Resistivity	205 nΩ·m at 20 °C
Magnetic Ordering	Paramagnetic
Young's Modulus	1.7 GPa
Bulk Modulus	1.6 GPa
Mohs Hardness	0.2
Brinell Hardness	0.14 MPa
Speed of Sound	1090 m/s (thin rod, RT)
Refractive Index	N/A (opaque metal)
Color	Silvery-gold
Luster	Metallic

Property	Value
Reactivity	Extremely reactive — ignites in air, explosive with water
Common Oxidation State	+1 (Cs⁺)
Electronegativity	0.79 (Pauling scale) — lowest of all stable elements
Ionization Energies	1st: 375.7 kJ/mol; 2nd: 2234.3 kJ/mol
Electron Affinity	45.51 kJ/mol
Standard Electrode Potential	−3.026 V (Cs⁺/Cs)
Reaction with Water	Cs + H₂O → CsOH + ½H₂ (violently exothermic)
Reaction with Air	Burns spontaneously; forms Cs₂O and Cs₂O₂
Common Compounds	CsCl, CsBr, CsI, CsOH, Cs₂CO₃, CsNO₃, CsF
CAS Number	7440-46-2
IUPAC Name	Caesium
Solubility (as CsCl in H₂O)	1865 g/L at 20 °C (highly soluble)
pH of CsOH solution	Strongly basic (pH > 13)
Flame Test Color	Blue-violet
Spectral Lines	455.5 nm, 459.3 nm, 894.3 nm (characteristic blue)
Coordination Number	Typically 8 in CsCl structure
Oxidation Potential (aqueous)	+3.026 V
Reduces	H₂O, O₂, halogens, acids — very strong reducing agent

Caesium has 40 known isotopes (mass numbers 112–151). Only Cs-133 is stable. Key isotopes are listed below.

Isotope	Mass (u)	Half-life	Decay Mode	Notes
Cs-133	132.90545	Stable	—	Only stable isotope; defines the SI second
Cs-134	133.90671	2.065 y	β⁻, EC, γ	Produced in nuclear reactors; environmental tracer
Cs-135	134.90600	2.3 × 10⁶ y	β⁻	Long-lived fission product
Cs-136	135.90731	13.16 d	β⁻, γ	Reactor product; used in calibration
Cs-137	136.90709	30.17 y	β⁻, γ (661.7 keV)	Significant fallout hazard; used in cancer radiotherapy
Cs-131	130.90546	9.69 d	EC, γ	Used in brachytherapy (prostate cancer)
Cs-129	128.90607	32.06 h	EC, γ	Used in SPECT imaging research
Cs-138	137.91101	33.41 min	β⁻, γ	Fission product; short-lived

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Summary

Caesium (symbol Cs, atomic number 55) is a soft, silvery-gold alkali metal and one of the most electropositive elements on the periodic table. It is highly reactive with water and air, melting just above room temperature at 28.5 °C. This reference card compiles all key physical, chemical, and nuclear data alongside notable real-world applications.

How it works

Select a property category using the tab buttons at the top.
Each panel displays a structured table of values with SI units.
Click any value or label to copy it to your clipboard.
Use the search box to jump directly to a specific property.
Open the Isotopes tab to compare decay modes, half-lives, and characteristics of major caesium isotopes.

Use cases

Quick reference for chemistry homework or exam revision.
Engineers designing atomic clocks (caesium frequency standard).
Researchers working with alkali metal chemistry.
Science educators preparing lesson materials on Group 1 elements.
Students comparing caesium to other alkali metals.
Technical writers needing verified SI-unit property values.
Nuclear scientists reviewing caesium-137 radiological data.
Lab technicians handling caesium compounds safely.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu

Caesium Element Properties

Caesium

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Caesium Element Properties

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Caesium

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Frequently Asked Questions