Chemical element · Atomic number 55
Cesium
Cesium in the periodic table: atomic number 55, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Alkali metal
solid
132.9054520 u
Documented element sampleMetallic caesium from a private element collection.
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Atomic classification
Shell occupancy
Cesium in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 18 electrons
- N · n=4
- 18 electrons
- O · n=5
- 8 electrons
- P · n=6
- 1 electron
- Electron configuration
- [Xe]6s1
- Electrons per shell
- 2 · 8 · 18 · 18 · 8 · 1
- Group
- 1
- Period
- 6
- Block
- S
- Element category
- Alkali metal
Physical and chemical properties
- Atomic mass
- 132.9054520 u
- Standard state
- solid
- Density
- 1.93 g/cm³
- Melting point
- 301.59 K
- Boiling point
- 944 K
- Electronegativity
- 0.79 (Pauling)
- First ionisation energy
- 3.894 eV
- Oxidation states
- +1
- Discovery
- 1860
Safety and periodic classification
Safety
Safe handling cannot be inferred from Cesium's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Cesium is in period 6, group 1 and the S block. Its direct neighbours by atomic number are Xenon and Barium. The recorded Pauling electronegativity is 0.79. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Cesium sits among the 118 elements
The table compares the reported values for Cesium with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Cesium | Position in the data set |
|---|---|---|
| Melting point | 301.59 K | Rank 89 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 944 K | Rank 76 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 1.93 g/cm³ | Rank 77 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 0.79 | Rank 94 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 3.894 eV | Rank 102 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Cesium in the periodic table
- Place in the table
- Group 1, period 6, S block. Classified as alkali metal, solid at standard conditions.
- Direct neighbours
- Atomic number 55. Preceded by Xenon, followed by Barium.
- Same group
- Same number of outer electrons, and therefore similar reactivity to Hydrogen, Lithium, Sodium, Potassium, Rubidium, Francium.
- Discovery
- Recorded in the snapshot as: 1860. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +1. They follow from the electron configuration [Xe]6s1 and indicate which compounds to expect.
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