Our Elements
Menu

Caesium

Element 55 · Alkali metal

Specimen

Atomic number
55
Atomic mass
132.91
State at room temperature
Solid
Melting point
28.44 °C
Boiling point
670.9 °C
Density
1.93 g/cm³
Discovered
1860

Atomic mass from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Not needed, little harm known. Food and water contain traces of stable cesium. ATSDR says people are unlikely to get enough to cause harm. Radioactive cesium in food from contaminated soil is a different danger. 34

See every element.

The atom

Cs
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 18
5 (O) 8
6 (P) outermost 1
Total55

Configuration [Xe] 6s¹

6s ↑
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 55 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Xe core, with one arrow per electron.

Related elements

Group 1
H Hydrogen Li Lithium Na Sodium K Potassium Rb Rubidium Fr Francium

New elements by e-mail

Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Abridged Standard Atomic Weights 2024 .

    The atomic mass shown for the 84 elements that have a standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    Agency for Toxic Substances and Disease Registry (ATSDR), 2004. ToxFAQs for Cesium .

    • Food and drinking water are the largest sources of exposure to cesium.
    • It is highly unlikely that you would be exposed to high enough amounts of stable cesium to cause harmful health effects.
  4. 4

    Royal Society of Chemistry, 2026. Caesium - Element information, properties and uses .

    • Caesium compounds, such as caesium chloride, are low hazard.
  5. 5

    National Institute of Standards and Technology (NIST), 2026. Atomic Weights and Isotopic Compositions for All Elements .

    Isotopic composition of each element; the most abundant isotope sets the neutron count in the nucleus view.