铯 (cesium) is a soft, silvery-gold alkali metal with the chemical symbol Cs and atomic number 55. It is highly reactive and is known for its low melting point, which is around 28.5 degrees Celsius. Due to its reactivity, it is usually stored in a vacuum or inert atmosphere to prevent it from reacting with moisture in the air. 铯 is primarily used in atomic clocks, which are the most accurate timekeeping devices available. These clocks rely on the vibrations of cesium atoms to measure time with incredible precision. Additionally, cesium is used in various applications, including photocells, ion propulsion systems, and medical imaging.