CS

Cesium (Cs) is a chemical element with atomic number 55 and is a soft, silvery-golden alkali metal in group 1 of the periodic table. It is among the most reactive of all metals, reacting vigorously with water and air. At room temperature it is near its melting point, m.p. 28.5°C, so it can be liquid in ordinary conditions, and it boils at about 671°C. The metal is typically stored under mineral oil to prevent oxidation.

The name cesium comes from the Latin caesius, meaning sky blue, in reference to the blue spectral line observed when it was discovered spectroscopically by Robert Bunsen and Gustav Kirchhoff in 1860. In nature, cesium is found only in trace amounts and is largely obtained from the mineral pollucite, a cesium-rich aluminosilicate. Commercial production involves separation and refining from pollucite or other complex minerals.

Cesium has several important applications. The most precise atomic clocks rely on the hyperfine transition of the stable isotope cesium-133, defining the international unit of time as the second. In the past, cesium vapor and compounds have been used to create low work function photocathodes for vacuum tubes and photomultiplier tubes, improving sensitivity in optical devices. Radioactive isotopes of cesium, such as cesium-137, have applications in medical therapy, industrial radiography, and sterilization, while others like cesium-131 are used in certain cancer therapies. Cesium remains of interest in research and specialized technologies due to its unique electronic and spectral properties, though its high reactivity requires careful handling and containment.