Cesium, with the chemical symbol Cs and atomic number 55, was first discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff. They isolated it from the mineral wateresite. The name "Cesium" is derived from the Latin "caesius," meaning "sky blue."

On Earth, cesium is found in small amounts, approximately 3 ppm in the Earth's crust. It is often found in association with minerals such as pollucite. Cesium has remarkable applications, especially in electronics, where it is used in photoelectric cells and magnetometers.

In nuclear physics, cesium played a disturbing role in the Chernobyl reactor disaster in 1986. Significant amounts of radioactive cesium, particularly cesium-137, were released during the accident. This isotope has a long half-life and contributes to the long-term contamination of the environment. The affected areas still grapple with the consequences of the catastrophe.

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