Cesium is a soft, highly reactive alkali metal. Its atoms provide a reference used in atomic clocks, connecting the periodic table to precise timekeeping and navigation. Cesium compounds have separate industrial roles, including drilling fluids and specialized optical glass, with behavior distinct from the metal.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
How can cesium atoms turn an oscillator into a reproducible time reference?
Cesium-133 atoms in an atomic clock
FOLLOW THIS RELATIONSHIP
Prepare the atomic reference
Cesium-133 atoms provide a specified transition between energy levels. The reference depends on atomic state differences rather than a mechanical part’s dimensions.
Explore the model (optional)
CsFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Atomic identity, reference properties and structure models
INTERACTIVE 3D55 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Xe] 6s1. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
Robert Bunsen and Gustav Kirchhoff recognized cesium in water in 1860 through unfamiliar blue lines. Their initial product was a compound, not a sample of pure metal. More than a century later, cesium-133 became central to the definition of the second. The connection is striking: reproducible atomic transitions first distinguished a new element and later provided a reference against which clocks could be compared.
1860
Spectral identification
Bunsen and Kirchhoff recognized cesium in mineral water.
1967
Second redefined
The SI second was defined using a cesium-133 atomic transition.
Cesium occurs in minerals such as pollucite and lepidolite, rather than as free metal in ordinary geological settings. Recovering it requires separating chemically bound constituents. The name on a mineral resource, a cesium salt, and an atomic-clock sample refers to the same element in different physical and chemical environments.
Cesium is a highly reactive alkali metal, so compounds dominate many practical uses. Atomic-clock operation instead examines a specific transition in cesium-133 atoms. Chemical reactivity involves electronic interactions; the label identifies the . The number 133 is a mass number, not a charge or a count of .
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
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Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
solid
293.15 K, 101.325 kPa
Melting point in K
301.59 K
at standard pressure unless noted
Boiling point in K
944 K
101.325 kPa
Density in g/cm³
1.87 g/cm³
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
0.79 Pauling
Pauling scale
First ionization energy in kJ/mol
375.7 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
244 pm
single-bond covalent radius
Common oxidation states
+1
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust in mg/kg
3 mg/kg
upper continental crust, mass fraction
Has stable isotopes
Yesderived
Discovery year in CE
1860 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 6
Cs 0.79 Pauling · Cesium
Electronegativity across period 6. Exact values.
Cs
0.79
Ba
0.89
La
1.1
Ce
1.12
Pr
1.13
Nd
1.14
Sm
1.17
Eu
1.2
Gd
1.2
Tb
1.1
Dy
1.22
Ho
1.23
Er
1.24
Tm
1.25
Yb
1.1
Lu
1.27
Hf
1.3
Ta
1.5
W
2.36
Re
1.9
Os
2.2
Ir
2.2
Pt
2.28
Au
2.54
Hg
2
Tl
1.62
Pb
2.33
Bi
2.02
Po
2
At
2.2
Electronegativity down group 1
Cs 0.79 Pauling · Cesium
Electronegativity down group 1. Exact values.
H
2.2
Li
0.98
Na
0.93
K
0.82
Rb
0.82
Cs
0.79
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
