Periodic table / Alkali metals
55Caesium
Caesium is an alkali metal in group 1, period 6. At 25 °C it is a solid.
- Symbol
- Cs
- Atomic number
- 55 protons in the nucleus
- Standard atomic weight
- 132.91 ± 0.01
- Electron configuration
- [Xe] 6s1
- Electrons in each shell
- 2, 8, 18, 18, 8, 1
- Group, period, block
- 1, 6, s
- Family
- Alkali metals
- State at 25 °C
- solid
- Electronegativity
- 0.79 (Pauling scale)
- First ionisation energy
- 375.7 kJ/mol (3.89390572743 eV)
Where these numbers come from
Atomic weight: IUPAC CIAAW, abridged standard atomic weights 2024. Electron configuration and ionisation energy: NIST Atomic Spectra Database. Family, state and electronegativity: PubChem. See Sources. Melting points, boiling points and densities are left out for now: published values differ between sources, and we will add them once each has been checked.