55132.91CsCaesium
Caesium (Cs) — element 55alkali metal · group 1, period 6
Caesium is a chemical element with the symbol Cs and atomic number 55. It is an alkali metal in group 1, period 6 of the periodic table (the s-block), with a standard atomic mass of 132.91 u. At room temperature (25 °C) caesium is a solid. It is also spelled cesium.
💡 Known for: Defines the second in atomic clocks; one of the most reactive metals, it melts at just 28.5 °C.
# Caesium properties
| Symbol | Cs |
|---|---|
| Atomic number | 55 |
| Atomic mass | 132.91 u |
| Category | alkali metal |
| Group | 1 |
| Period | 6 |
| Block | s-block |
| Electron configuration | [Xe] 6s¹ |
| Full configuration | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹ |
| Electrons per shell | 2, 8, 18, 18, 8, 1 |
| Protons / electrons | 55 / 55 |
| State at 25 °C | solid |
| Melting point | 301.59 K (28.4 °C, 83.2 °F) |
| Boiling point | 944 K (670.9 °C, 1239.5 °F) |
| Density | 1.873 g/cm³ |
| Electronegativity (Pauling) | 0.79 |
| First ionization energy | 3.894 eV |
| Atomic radius | 298 pm |
| Discovered | 1860 |
# Electron configuration of caesium
The electron configuration of caesium is [Xe] 6s¹, written out in full as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹. Its 55 electrons fill 6 shells as 2, 8, 18, 18, 8, 1, so the outermost shell holds 1 electron. As a group 1 element, caesium has 1 valence electron.
# Related elements
XeXenonprevious elementBaBariumnext elementHHydrogengroup 1LiLithiumgroup 1NaSodiumgroup 1KPotassiumgroup 1RbRubidiumgroup 1FrFranciumgroup 1
# Caesium FAQ
What is the atomic number of caesium?
The atomic number of caesium is 55: every caesium atom has 55 protons in its nucleus. Its symbol is Cs.
What is the atomic mass of caesium?
The standard atomic mass of caesium is 132.91 u.
What is the electron configuration of caesium?
Caesium's electron configuration is [Xe] 6s¹ (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹), with 2, 8, 18, 18, 8, 1 electrons per shell.
Is caesium a metal?
Yes. Caesium is an alkali metal, one of the metals.
What is the melting point of caesium?
Caesium melts at 301.59 K (28.4 °C, 83.2 °F) and boils at 944 K (670.9 °C, 1239.5 °F).
What is caesium used for / known for?
Defines the second in atomic clocks; one of the most reactive metals, it melts at just 28.5 °C.