Caesium (Cs): Electron Configuration, Properties and Uses
Caesium (Cs): Electron Configuration, Properties and Uses
Electron configuration [Xe] 6s1 · group 1 · period 6 · Ar 132.91
Caesium data
- Atomic number
- 55
- Relative atomic mass
- 132.91
- Group
- 1
- Period
- 6
- Block
- s
- Category
- Alkali metal
- Electron configuration
- [Xe] 6s1
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1
- Electrons per shell
- 2, 8, 18, 18, 8, 1 (K L M N O P)
- Valence electrons
- 1
- Electronegativity (Pauling)
- 0.79
- First ionisation energy
- 3.894 eV
- Atomic radius (van der Waals)
- 343 pm
- State at room temperature
- Solid
- Melting point
- 28.4 °C
- Boiling point
- 671 °C
- Density
- 1.93 g/cm³
- Common oxidation states
- +1
- Discovered
- 1860
Source: PubChem (NIH, accessed 13 Nov 2026); atomic mass: IUPAC standard atomic weight.
Electron shell diagram
K 2 · L 8 · M 18 · N 18 · O 8 · P 1
Orbital diagram (outer electrons)
About caesium
With atomic number 55, caesium (Cs) has 55 protons in its nucleus and 55 electrons as a neutral atom. It is an alkali metal in group 1, period 6, s-block. The element was discovered in 1860. Caesium is solid at room temperature, but it melts at only 28.4 °C, so it can melt in your hand.
Caesium electron configuration
The electron configuration of caesium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1, or in short [Xe] 6s1. Its electrons occupy 6 shells: K = 2, L = 8, M = 18, N = 18, O = 8, P = 1. Caesium has 1 valence electron in the P shell (6s1), which is why it sits in group 1. When it forms cations, electrons leave the outermost subshell first: Cs+ = [Xe] (like xenon).
Filling order: 1s2(2) → 2s2(4) → 2p6(10) → 3s2(12) → 3p6(18) → 4s2(20) → 3d10(30) → 4p6(36) → 5s2(38) → 4d10(48) → 5p6(54) → 6s1(55)
Uses of caesium
In industry and the laboratory, caesium turns up in many places, for example caesium atomic clocks that define the SI second, caesium formate drilling fluids and photocells. For research and analysis you can browse caesium compounds in the AMI Scientific catalogue.
Safety
Like the other alkali metals, caesium reacts violently with water and air, so it is stored under oil or inert gas. Always read the safety data sheet (SDS) before working with a chemical.
Other elements in group 1
Frequently asked questions
What is the electron configuration of caesium?
The electron configuration of caesium (Cs, atomic number 55) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1, or [Xe] 6s1 in short. By shell: 2, 8, 18, 18, 8, 1.
How many valence electrons does caesium have?
Caesium has 1 valence electron.
Which group and period is caesium in?
Caesium is in group 1, period 6, s-block.
What is the atomic mass of caesium?
The relative atomic mass of caesium is 132.91. Use it to calculate molar mass with the molar mass calculator.
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