55 Cs Cesium 132.905
Alkali metal s-block Period 6 Group 1

Cesium

Cs · Element 55 · Alkali metal

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The most reactive metal, the softest, and the atom that defines the second.

State at 20°C Solid
Atomic mass 132.905 u
Electron configuration[Xe] 6s¹
54Xe All 118 56Ba

Structure

The cesium atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where cesium sits among all 118 elements for that property.

Physical

Density 1.93 g/cm³ 17%
Melting point 301.6 K 15%
Boiling point 944 K 21%
Specific heat 0.242 J/g·K
Thermal conductivity 35.9 W/m·K 58%

Atomic

Atomic radius 265 pm 100%
Covalent radius 244 pm
Van der Waals radius 343 pm

Electronic

Electronegativity 0.79 1%
Ionisation energy 375.3 kJ/mol 0%
Electron affinity 45.3 kJ/mol 37%

Occurrence

Abundance in crust 3 mg/kg 62%

Identity

SymbolCs
Atomic number55
Atomic mass132.9055 u
CategoryAlkali metal
Blocks
Crystal structurebody-centered cubic
Oxidation states-1, +1
Discovered1860
Discovered byBunsen & Kirchhoff

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Same element, different neutrons

Isotopes of cesium

All cesium atoms have the same number of protons, 55, and differ only in their number of neutrons: those variants are its isotopes. The most common, Cs-133, has 78 neutrons and makes up 100% of natural cesium.

Isotopes of cesium found in nature, with their share of the natural element, and well-known radioactive ones
ISOTOPE Neutrons Natural Abundance Half-life Decay Mode
Cs-133 most common 78 100% Stable —
Cs-134 79 — 2.0652 yrs β⁻
Cs-137 82 — 30.08 yrs β⁻

Here the classroom shortcut, rounded atomic mass (132.905) minus atomic number, gives the same answer: 78.

Decay modes

β⁻
beta-minus decay

Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.

Size, to scale

How big is a cesium atom?

Radius 265 pm — that is 0.265 nm, so about 1.89 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Cesium is liquid over a 642 K window, from 302 K to 944 K.

Where it sits

Position in the table

Cesium sits in period 6, group 1. Everything in group 1 shares the same outer-electron count, which is why they behave so similarly.

Other alkali metals

3Li 11Na 19K 37Rb 87Fr
All alkali metals

The story

What cesium is, and how we found it

Cesium (Cs) is element 55, a soft, pale gold alkali metal in group 1. It melts at 28.44 °C (301.59 K), just above room temperature, so it can turn liquid on a warm day, and it boils at 671 °C (944 K). Its density is 1.93 g/cm³. Cesium is the most electropositive stable element: it gives up its outer electron so easily that it ignites in air and reacts explosively with water. IUPAC spells the name caesium; cesium is the American spelling.

Atomic structure and isotopes

Cesium has 55 electrons in six shells, arranged 2, 8, 18, 18, 8, 1, with the configuration [Xe] 6s¹. Its single outer electron is far from the nucleus, which makes the cesium atom one of the largest of all atoms.

Natural cesium is a single stable isotope, cesium-133, which is used to define the second. The radioactive isotopes cesium-137 (half-life about 30 years) and cesium-134 are products of nuclear fission and were released by the Chernobyl and Fukushima accidents.

The discovery of cesium

Found with the spectroscope (1860)

In 1860 Robert Bunsen and Gustav Kirchhoff in Heidelberg evaporated large volumes of mineral water from Dürkheim and examined the flame of the residue with their new spectroscope. They saw two bright blue lines that matched no known element.

The name

They named the element after the Latin caesius, sky blue, for the color of those lines. Cesium was the first element found with the spectroscope; rubidium followed in 1861.

The metal (1882)

Bunsen could not isolate the metal. Carl Setterberg first obtained it in 1882 by electrolysis of molten cesium cyanide.

Applications

What cesium is used for

Atomic clocks

Since 1967 the second has been defined by cesium-133: it is the duration of 9,192,631,770 periods of the radiation emitted in a transition between two energy levels of the cesium atom. Cesium atomic clocks keep national time standards, and GPS and other navigation satellites carry cesium and rubidium clocks.

Catalysts and drilling fluids

By quantity, cesium carbonate and cesium hydroxide, used mainly as catalysts, are the leading cesium products, followed by cesium formate brine, a dense, reusable fluid used when drilling high-pressure, high-temperature oil and gas wells (USGS).

Other uses

  • Electronics and optics: cesium is used in photoelectric cells, photomultiplier tubes and night vision devices.
  • Chemistry: cesium compounds serve as catalysts and in specialty glasses.
  • Radiation sources: cesium-137 has been used in cancer radiotherapy, blood irradiators and industrial gauges, though X-ray sources are replacing it because of security concerns.

In everyday life

Cesium works mostly out of sight. When a phone sets its clock from the network or a GPS receiver finds its position, it relies on time kept by cesium atomic clocks. Most people never handle the metal itself, which is sold sealed in glass ampoules.

Where it comes from

Natural occurrence

3 mg/kg of Earth's crust · more abundant than 62% of elements

Abundance

Cesium makes up about 3 parts per million of Earth's crust. It is too reactive to occur free, and it is widely dispersed except in a few unusual granite pegmatites.

Minerals

The main ore is pollucite, a cesium aluminum silicate. The largest known deposit was at the Tanco mine at Bernic Lake in Manitoba, Canada; other deposits are at Bikita in Zimbabwe and Sinclair in Australia. Cesium is also present in lepidolite and other lithium minerals and in some mineral waters.

Production

World production is small, comes from a few mines, and is not reported in detail (USGS).

Handling

Safety

Reactivity

Cesium metal reacts explosively with water, producing hydrogen and cesium hydroxide, one of the strongest bases, and it can ignite on contact with air. It must be stored under dry inert gas or mineral oil in sealed containers, and fires must be fought with a Class D extinguisher or dry sand, never water.

Toxicity

Stable cesium compounds have low toxicity; the body handles cesium much like potassium. Cesium hydroxide, however, is highly corrosive.

Radioactive cesium

Cesium-137 is a serious radiation hazard. Through its short-lived decay product it emits gamma radiation, and inside the body it spreads like potassium. In 1987 in Goiânia, Brazil, an abandoned radiotherapy source containing cesium-137 chloride was opened, killing four people and contaminating many others.

Keep exploring

Use cesium in the tools

Quick answers

Cesium: common questions

What is cesium?

Cesium (symbol Cs) is element 55 on the periodic table, an alkali metal in period 6, group 1. The most reactive metal, the softest, and the atom that defines the second. At room temperature it is a solid.

What is the electron configuration of cesium?

Cesium's ground-state electron configuration is [Xe] 6s¹, giving 6 occupied shells with the distribution 2, 8, 18, 18, 8, 1.

What are the melting and boiling points of cesium?

Cesium melts at 301.6 K (28.4 °C) and boils at 944 K (670.9 °C). It will melt in a warm room or in the palm of your hand.

What is the atomic mass of cesium?

The standard atomic weight of cesium is 132.905 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How many neutrons does cesium have?

The most common cesium atom, Cs-133, has 78 neutrons: its mass number, 133, minus its atomic number, 55. It makes up 100% of natural cesium. The classroom shortcut, rounded atomic mass (132.905) minus atomic number, gives the same 78.

How dense is cesium?

Cesium has a density of 1.93 g/cm³. Water is 1.0 g/cm³, so a block of cesium is about 1.9× heavier.

What is the electronegativity of cesium?

Cesium has a Pauling electronegativity of 0.79. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this low means it readily gives its outer electrons away, forming positive ions.

Who discovered cesium, and when?

Cesium was discovered in 1860 by Bunsen & Kirchhoff. Origin of the name: Latin caesius, "sky blue".

How common is cesium on Earth?

Cesium makes up about 3 mg/kg of the Earth's crust — uncommon, but not rare.

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