Cs Caesium

Quick answerCaesium (Cs) is an alkali metal with atomic number 55 and an atomic mass of 132.91 (2023), in group 1, period 6. It was discovered by Robert Bunsen and Gustav Kirchhoff in 1860. It is used for atomic clocks and oil drilling fluids. Its electron configuration is [Xe] 6s1.

Caesium is the soft gold metal that defines the second in atomic clocks, the most accurate timekeepers ever built.

Key facts

Symbol	Cs
Atomic number	55
Atomic mass	132.91 (IUPAC standard atomic weight, 2023)
Category	alkali metal
Group	1
Period	6
Block	s-block
Electron configuration	[Xe] 6s1
Discovered	Robert Bunsen and Gustav Kirchhoff, 1860
Uses	atomic clocks, oil drilling fluids, medical radiotherapy, photoelectric cells, GPS timing

Quick quiz

Three questions from the facts on this page. No sign-up, no score is saved.

1. What is the chemical symbol of Caesium?

Li Cs K Na

2. What is the atomic number of Caesium?

54 57 56 55

3. Which category does Caesium belong to?

alkali metal halogen noble gas transition metal

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Keep exploring

• Periodic table — all 118 elements
• Lithium (Li) — another alkali metal, atomic number 3
• Sodium (Na) — another alkali metal, atomic number 11
• Potassium (K) — another alkali metal, atomic number 19
• Rubidium (Rb) — another alkali metal, atomic number 37
• Francium (Fr) — another alkali metal, atomic number 87

Sources & last updated

• CIAAW — Standard Atomic Weights
• PubChem Periodic Table
• Wikipedia — Timeline of chemical element discoveries

Last updated: 2026-11-12. Figures are shown with their year; where sources disagree, the value is left out rather than estimated.