cesium

Cesium (Cs), atomic number 55, is a highly reactive alkali metal. It is soft and silvery-golden, with a melting point of 28.5°C and a density of 1.93 g/cm3. It reacts vigorously with water and air, forming oxides, hydroxides, and peroxides.

In nature, cesium is rare in the Earth's crust, occurring only in trace amounts and primarily in the mineral pollucite (CsAlSi2O6). Commercial production comes from pollucite-bearing ore and is processed to yield cesium salts or metal. Major mining and production have occurred in Canada and Namibia, among other locations.

The most important isotopes are Cs-133, which is stable, and Cs-137, a radioactive product of nuclear fission. Cesium-133's hyperfine transition underpins modern timekeeping in atomic clocks, and cesium vapor devices and photocathodes also employ cesium. Cesium formate brines are used in high-density drilling fluids for oil and gas exploration.

Applications span scientific, medical, and industrial fields. Cs-137 is used in radiotherapy and industrial gauges, while stable cesium and its compounds find use in vacuum technology, photoemissive devices, and specialty drilling fluids. The element is also involved in research on ultracold atoms and fundamental physics experiments.

Handling cesium requires care because of its reactivity with moisture and air; it is typically stored under oil or in an inert atmosphere. The name derives from the Latin caesius, meaning sky-blue, in reference to blue spectral lines observed when cesium was discovered by Bunsen and Kirchhoff in 1860.