Cesium (Cs)
Element 55 · Alkali metal · atomic mass 132.91 u · group 1, period 6
Electron shells: 2, 8, 18, 18, 8, 1
Cesium is the most reactive metal and melts barely above room temperature. Its hyperfine transition is the official definition of the second.
One second is defined as exactly 9,192,631,770 oscillations of a caesium-133 atom.
- Atomic number
- 55
- Atomic mass
- 132.91 u
- Electron configuration
- [Xe] 6s¹
- Group · period · block
- 1 · 6 · s
- Oxidation states
- +1
- Electronegativity
- 0.79 (Pauling)
- Melting point
- 28.44 °C
- Boiling point
- 671 °C
- Density
- 1.93 g/cm³
- State at 20 °C
- Solid
- Appearance
- Soft, silvery-gold metal
- Also spelled
- Caesium
- Discovered
- Robert Bunsen and Gustav Kirchhoff, 1860
55 protons, and 55 electrons when neutral
IUPAC standard atomic weight
Alkali metals
Both spellings are accepted; this page uses the one in the source data
What Cesium is used for
- Atomic clocks and GPS timing
- Oil-drilling fluids
- Photoelectric cells
- Vacuum tube getters
Atomic structure of Cesium
A neutral cesium atom has 55 protons and 55 electrons. Its most common isotope carries about 78 neutrons, which is where the mass number 133 comes from. Change the neutron count and you have a different isotope of cesium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 18, 8, 1. In orbital notation that is [Xe] 6s¹, leaving 1 valence electron in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Cesium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Cesium sits in the periodic table
Group 1 metals with a single outer electron. Soft, low-melting and so reactive they are stored under oil — reactivity increases as you go down the group.
Discovery and naming
Cesium was discovered by Robert Bunsen and Gustav Kirchhoff in 1860. Its name comes from Latin caesius, "sky blue", which is also why its symbol (Cs) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Cesium?
The standard atomic weight of Cesium is 132.91 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Cesium have?
Cesium has 55 protons, because the atomic number is the proton count and that is what makes an atom Cesium at all. A neutral Cesium atom has the same number of electrons, 55. The neutron count varies by isotope; the most common one has about 78, since 133 − 55 = 78.
What is the electron configuration of Cesium?
The electron configuration of Cesium is [Xe] 6s¹. Its electrons are arranged 2, 8, 18, 18, 8, 1 across 6 shells.
Is Cesium a metal?
Yes. Cesium is classified as a alkali metal, on the left side of the table.
What group and period is Cesium in?
Cesium is in group 1 (the alkali metals), period 6, in the s-block. The group is its column and the period is its row.
What is the charge of Cesium?
Cesium most commonly forms a 1+ ion. It has 1 electron in its outer shell and loses it readily, which is what the group number tells you.
What is the melting point of Cesium?
Cesium melts at 28.44 °C and boils at 671 °C. That puts it in the solids at everyday temperatures.
What is Cesium used for?
Atomic clocks and GPS timing, Oil-drilling fluids, Photoelectric cells, Vacuum tube getters. Cesium is the most reactive metal and melts barely above room temperature.
Who discovered Cesium, and where does its name come from?
Cesium (Cs) was discovered by Robert Bunsen and Gustav Kirchhoff in 1860. The name comes from Latin caesius, "sky blue".