Cesium

01 / A closer look

Meet Cesium

Cesium has atomic number 55: every atom of this element contains 55 protons. It belongs to period 6, group 1 of the periodic table.

In a neutral atom, the number of electrons equals the number of protons. Isotopes of this element share the same atomic number but have different neutron counts.

02 / Physical properties

Melting point

301.7 K

Boiling point

944 K

Density

1.873 g/cm³

Atomic weight

132.91 ± 0.01

Melting points, boiling points and densities follow RSC fact boxes. Density is referenced to room temperature; consult the source for pressure and allotrope details. These are reference values, not a claim that every value was experimentally measured. Atomic weights follow CIAAW.

Physical-property source: Royal Society of Chemistry ↗ · Reviewed 24 September 2026

Standard atomic weights follow CIAAW 2024 (abridged). Brackets show an isotope mass number from IUPAC 2022, not a standard atomic weight.

03 / Atomic structure

Atomic structure

Electron configuration

[Xe]6s1

Oxidation states

+1

Electronegativity · Pauling

0.79

First ionisation energy

3.894 eV

Atomic radius

343 pm

04 / Where it is found

Where it is found

Pollucite is an important caesium mineral.

Production and export figures depend on the commodity and reporting year. No comparable country quantities have been included for this element in this edition.