CäsiumAtoms

Cäsium atoms are the atoms of the chemical element cesium, symbol Cs, and atomic number 55. They belong to the alkali metal group and are among the softest and most reactive metals. Cesium is a silvery-golden metal with a low melting point of about 28.5 °C, so it can melt in a warm room. Because of its high reactivity with air and water, it is typically stored under mineral oil or in inert environments.

In the ground state, cesium has the electronic configuration [Xe] 6s1, giving it one valence electron that is readily donated to form Cs+ ions. This results in strong reactivity, especially with water, producing hydrogen gas and caustic cesium hydroxide. Cesium metal readily forms compounds with many nonmetals and participates in various alloy and semiconductor processes.

Natural cesium is overwhelmingly Cs-133, a stable isotope. Several radioactive isotopes have been produced synthetically, notably Cs-134 and Cs-137, which have half-lives of about 2 years and 30 years, respectively. These radioisotopes have been used in medical, industrial, and scientific applications, though they require strict handling and shielding.

Natural occurrence of cesium is in trace amounts in the Earth's crust, often extracted from minerals such as pollucite. Major producers include several mining regions worldwide. Cesium compounds have historical and contemporary uses in electronics, chemistry, and metrology, with Cs-133 hyperfine transitions forming the basis of the definition of the second in atomic clocks.

The element is named after the Latin caesius, meaning blue-gray or sky-blue, reflecting its spectral lines observed in early spectroscopy.