Cesium - Cs

Cesium (caesium) — chemical properties and water context

Cesium (also spelled caesium) is a soft, gold-coloured alkali metal. Its standard atomic weight is based on the natural isotopic composition; the values below describe elemental cesium, not a cesium compound.

Atomic number	55
Standard atomic weight	132.90545196 g/mol
Density	1.873 g/cm3
Melting point	28.5 °C
Boiling point	671 °C
Electron configuration	[Xe] 6s1

Chemical properties

Cesium is in Group 1 and Period 6. It has one outer-shell electron and commonly forms the cesium ion, Cs⁺. The metal melts just above room temperature and reacts very vigorously with water, so it must be handled under controlled conditions. Cesium is found in minerals such as pollucite; it is not encountered in nature as free metal.

Uses and applications

Cesium compounds are used in specialized applications, including drilling fluids, optical materials and as catalysts. The stable isotope cesium-133 is also used in atomic clocks. Radioactive cesium isotopes, such as cesium-137, are distinct from stable cesium and require radiological controls.

Cesium in water treatment

Dissolved cesium is generally present as Cs⁺. Ion exchange and membrane processes can remove alkali-metal ions from water, but performance depends on salinity, competing ions and the target concentration. Radioactive cesium requires a radiological assessment and treatment approach selected for the isotope and water matrix; routine water-treatment guidance should not be used as a substitute for that assessment.

Environmental and health context

Stable cesium and radioactive cesium have different radiological significance, although their chemistry is similar. Cesium compounds can dissolve in water, while cesium can bind strongly to some soil minerals. Exposure assessment should distinguish total cesium from radioactive isotopes and follow applicable radiation-protection procedures.

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