PeriodicElementTable.com
55Cs132.905452
Alkali metal

Cesium

Cesium (Cs) is element 55, an alkali metal in group 1, period 6, with an atomic mass of 132.905452 u. It is a solid at room temperature and melts at 28.44 °C. Its electron configuration is [Xe] 6s¹.

Cesium is a soft, gold-tinged alkali metal so reactive that it bursts into flame or explodes on contact with water, and so soft-melting that it can turn to liquid in a warm room. It's best known today for a very different reason: the vibrations of a cesium atom are so regular that they're used to define the length of a second itself.

Group · Period
1 · 6
At room temp
solid
Melts at
301.59 K
Density
1.93 g/cm³
Discovered
1860

Key facts about cesium

Quick answers, built from the sourced data in the property tables.

What is the electron configuration of cesium?
The electron configuration of cesium is [Xe] 6s¹. Written out in full, that is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹.
What is the density of cesium?
The density of cesium is 1.93 g/cm³.
What are the melting and boiling points of cesium?
Cesium melts at 301.59 K (28.44 °C, 83.19 °F) and boils at 944 K (670.85 °C, 1239.53 °F).
What is the electron affinity of cesium?
The electron affinity of cesium is 0.472 eV, which is about 45.5 kJ/mol.
How many stable isotopes does cesium have?
Cesium has one stable isotope: Cs-133 (100%).

Uses

Cesium’s defining modern use is in atomic clocks, where the precise, unchanging frequency of its atoms provides the reference against which the international second is defined; these clocks underpin GPS satellite navigation and global timekeeping standards. In the oil and gas industry, cesium formate is used in dense drilling fluids that help control pressure and stabilize wells during deep drilling operations.

Cesium compounds also find use in specialized photoelectric cells, which generate a small electric current when exposed to light, taking advantage of how easily cesium atoms give up an electron.

History

Cesium was discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff, pioneers of a then-new technique called flame spectroscopy that identified elements by the distinctive colors of light they emit when heated. While analyzing mineral water samples, they spotted two previously unrecorded blue spectral lines and used them to identify a new element, naming it cesium after the Latin word caesius, meaning sky-blue, in reference to those lines. It remained largely a laboratory curiosity until the mid-twentieth century, when its extraordinarily precise atomic vibrations made it the foundation of modern atomic timekeeping.

Fun facts

  • The SI second is officially defined by counting a precise number of oscillations of a cesium-133 atom, making cesium atomic clocks the world's timekeeping standard.
  • Cesium melts at only about 28°C, just above room temperature, making it one of the few metals that can turn to liquid in your hand.
  • Cesium reacts explosively with water, even more violently than sodium or potassium, the more commonly seen reactive alkali metals.

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Frequently asked questions

Why is cesium used to define the length of a second?

A cesium-133 atom absorbs and emits radiation at an extremely precise, unchanging frequency when it switches between two specific energy states. That frequency is so stable and repeatable that scientists chose it as the reference standard, and cesium atomic clocks built around it are accurate enough to stay within a second over many millions of years.

Is cesium metal actually dangerous to handle?

Very. Cesium reacts violently and often explosively with water, including the moisture in air, so pure cesium metal has to be stored sealed away from air entirely, typically under inert gas or oil. Its extreme reactivity is part of a broader pattern among the alkali metals, which get more reactive the further down the group you go.

Why does cesium melt so easily compared with other metals?

Cesium's atoms are large and hold onto their outer electron only loosely, which means the metallic bonds tying its atoms together in a solid are unusually weak. That weak bonding is why cesium melts at just under 30°C, making it one of only a handful of metals — alongside gallium, rubidium, and mercury (which is liquid well below room temperature) — that are liquid at or near everyday room temperature.

Compounds

5 notable compounds containing Cs

  • Cs2CO3salt

    Cesium carbonate

    A hygroscopic white salt widely used as a mild, highly soluble base in organic and pharmaceutical synthesis.

    Used for: Base reagent in organic synthesis, including palladium-catalyzed couplings

  • CsClsalt

    Cesium chloride

    A simple ionic salt whose crystal structure gives its name to a common packing arrangement, and whose dense solutions are used to separate biomolecules by density.

    Used for: Density-gradient medium for separating DNA and other biomolecules

  • CsFhalide

    Cesium fluoride

    A very water-soluble ionic salt that serves as a source of "naked" fluoride ion in organic synthesis, thanks to cesium's weak attraction to it.

    Used for: Fluoride source and mild base in organic synthesis

  • CsOHbase

    Cesium hydroxide

    The strongest of the common alkali metal hydroxides, a highly corrosive, deliquescent solid that dissolves readily in water.

    Used for: Strong base used in specialty batteries and research chemistry

  • CsIsalt

    Cesium iodide

    A dense ionic salt that, when doped with thallium, forms crystals that scintillate brightly under radiation and are widely used in detectors.

    Used for: Scintillation crystals in radiation and medical imaging detectors

Isotopes

41 known isotopes

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IsotopeNatural abundanceHalf-lifeDecay modeMass excessBinding energy / nucleonDecay widthSpin/parityMagnetic momentQuadrupole moment
Cs-133 stable 100% Stable — -88,070.94 keV 8,409.98 keV — 7/2+ 2.5778 μN -0.00343 b

Spin/parity describes the nucleus's intrinsic angular momentum and symmetry. Magnetic moment (in nuclear magnetons, μN) and quadrupole moment (in barns, b) describe the shape of the nucleus's charge distribution — a nonzero quadrupole moment means the nucleus isn't perfectly spherical. These are only measured for a subset of isotopes, mostly stable or long-lived ones, so many rows show "—" rather than a value.