Cesium (Cs) – element 55
Cesium is a chemical element with the symbol Cs and atomic number 55. It is a alkali metal in group 1 (alkali metals), period 6 of the periodic table (s-block), and is a solid at room temperature.
Cesium (Cs) has atomic number 55, atomic mass 132.91 u and electron configuration [Xe] 6s1.
Cesium facts
| Symbol | Cs |
|---|---|
| Atomic number | 55 |
| Atomic mass | 132.91 u (g/mol) |
| Category | alkali metal |
| Group | group 1 (alkali metals) |
| Period | 6 |
| Block | s-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1 |
| Noble-gas notation | [Xe] 6s1 |
| Electrons per shell | 2, 8, 18, 18, 8, 1 |
| Valence electrons | 1 |
Bohr model and shells
Cesium has 55 electrons in 6 shells: 2, 8, 18, 18, 8, 1. Full configuration and noble-gas notation are in the table; work out any element or ion with the electron configuration calculator.
Physical and atomic properties
| Melting point | 301.7 K (28.6 °C) |
|---|---|
| Boiling point | 944 K (670.9 °C) |
| Density | 1.93 g/cm³ |
| Electronegativity (Pauling) | 0.79 |
| Electron affinity | 45.505 kJ/mol |
| Ionization energies | 375.7, 2234.3, 3400 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 32.21 J/(mol·K) |
| Discovered by | Robert Bunsen |
Use the atomic mass in the molar mass calculator for any formula containing Cs.
Position in the periodic table
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Frequently asked questions
What is the atomic mass of cesium?
The atomic mass of cesium is 132.91 u, so one mole of cesium atoms weighs 132.91 g.
What is the electron configuration of cesium?
Cesium has 55 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹, or [Xe] 6s¹ in noble-gas notation. Electrons per shell: 2, 8, 18, 18, 8, 1.
How many valence electrons does cesium have?
Cesium has 1 valence electron (group 1).
Is cesium a metal?
Yes. Cesium is a alkali metal.
Where is cesium on the periodic table?
Cesium is in group 1 (alkali metals), period 6, in the s-block.
Data: Periodic-Table-JSON (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.