Element 55 · Alkali metal
Caesium (Cs) — Element 55
Latin caesius — sky blue
Caesium (Cs) is an alkali metal in Period 6, Group 1 of the periodic table with atomic number 55, standard atomic weight 132.91 u, and ground-state electron configuration [Xe] 6s¹. At standard temperature and pressure it is classified as a solid with 1 valence electron and common oxidation states +1.
Atomic clocks, drilling fluids
What are the atomic properties and physical constants of Caesium?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 55
- Standard atomic mass
- 132.91 u
- Chemical family
- Alkali metal
- Ground-state configuration
- [Xe] 6s¹
- Electrons per shell
- 2, 8, 18, 18, 8, 1
- Valence electrons
- 1
- Common ion charges / oxidation states
- +1
- Phase at STP
- Solid
- Pauling electronegativity
- 0.79 Pauling
- First ionization energy
- 376 kJ/mol
- Atomic radius
- 298 pm
- Melting point
- 29 °C
- Boiling point
- 671 °C
- Discovery year
- 1860
- Table coordinate
- Period 6, Group 1
- Radioactivity
- Has stable isotope(s)
How are electrons arranged across Caesium’s shells?
A neutral Caesium atom contains 55 electrons distributed across 6 occupied principal energy shells ([Xe] 6s¹). The table below compares Caesium’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Cs | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 18 e⁻ | 18 e⁻ |
| n = 4 | N | 18 e⁻ | 32 e⁻ |
| n = 5 | O | 8 e⁻ | 50 e⁻ |
| n = 6 | P | 1 e⁻ | 72 e⁻ |
Frequently Asked Questions about Caesium (Cs)
What is the atomic mass and atomic number of Caesium (Cs)?
Caesium (Cs) has atomic number Z = 55 (55 protons in its nucleus) and an IUPAC standard atomic weight of 132.91 u. It sits in Period 6, Group 1 and is classified as an alkali metal.
What is the ground-state electron configuration of Caesium?
The neutral ground-state electron configuration of Caesium (Z = 55) is [Xe] 6s¹. Across its 6 occupied principal shells (n = 1 to n = 6), the electrons per shell are 2, 8, 18, 18, 8, 1, with 1 valence electron participating in bonding.
What are the common ion charges and electronegativity of Caesium?
Caesium commonly forms +1 oxidation states or ion charges in compounds. Its Pauling electronegativity is 0.79 and its first ionization energy is 376 kJ/mol.
When was Caesium discovered and what is it used for?
Caesium was discovered in 1860 (Latin caesius — sky blue). Atomic clocks, drilling fluids
How does Caesium compare to neighboring elements?
Other alkali metal elements: