Cesium (Cs)
Alkali metal
Caesium is a gold-tinted alkali metal that melts just above room temperature and reacts explosively with water. Its claim to fame is timekeeping: the second is defined by the vibration of caesium-133 atoms.
Position on the periodic table
Atomic properties
| Atomic number | 55 |
|---|---|
| Atomic mass | 132.905 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1 |
| Noble gas shorthand | [Xe] 6s1 |
| Electrons per shell | 2, 8, 18, 18, 8, 1 |
| Valence electrons (outer shell) | 1 |
| Common oxidation states | +1 |
| Electronegativity (Pauling) | 0.79 |
| Covalent radius (approx.) | 244 pm |
| First ionization energy | 375.7 kJ/mol |
| Electron affinity | 45.5 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 1.93 g/cm³ |
| Melting point | 28.55 °C (301.7 K) |
| Boiling point | 670.85 °C (944 K) |
| Appearance | silvery gold |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of cesium
Cesium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1, usually shortened to [Xe] 6s1. Its electrons occupy 6 shells (2, 8, 18, 18, 8, 1), placing it in period 6 of the s-block. The 1 outer-shell electron drives its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of cesium
- Atomic clocks that define the second
- Drilling fluids (caesium formate)
- Photoelectric cells
- Atomic physics research
Biological role: None; behaves loosely like potassium in the body.
Occurrence: Rare; mined chiefly from pollucite at Bernic Lake, Canada.
Common compounds of cesium
CsCl
Caesium chloride
CsOH
Caesium hydroxide
History and discovery
Discovered: 1860 — Robert Bunsen. Name origin: From Latin caesius, sky-blue, for its blue spectral lines.
Bunsen and Kirchhoff discovered caesium in 1860 — the first element found by spectroscope.
Safety notes
Explosively water-reactive; radioactive Cs-137 from accidents is a serious contaminant, unlike stable natural caesium.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Cesium
1. What is the chemical symbol of Cesium?
2. What is the atomic number of Cesium?
3. Which category does Cesium belong to?
4. What is the state of Cesium at room temperature?
5. Which period of the periodic table is Cesium in?
Cesium FAQs
What is the atomic number of cesium?
Cesium's atomic number is 55 — every cesium atom has 55 protons in its nucleus.
What is the symbol for cesium?
Cs. From Latin caesius, sky-blue, for its blue spectral lines.
Is cesium a metal, nonmetal, or metalloid?
Cesium is classified as a alkali metal.
What state is cesium at room temperature?
At about 25 °C, cesium is a solid. It melts at 28.55 °C.
How many valence electrons does cesium have?
Cesium has 1 electron in its outer shell (shell pattern: 2, 8, 18, 18, 8, 1).
What is cesium used for?
Key uses include: atomic clocks that define the second; drilling fluids (caesium formate); photoelectric cells.
Related elements
Keep working with this element
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