Caesium is an alkali earth metal, characterized by having the symbol Cs and an atomic number of 55. It is known for its high reactivity and low ionization energy, making it a strong reducing agent. Physically, it is a soft, silvery-metal that is highly reactive with air and moisture, typically requiring storage under inert mineral oil. Due to its unique electronic properties, the element is utilized in the field of physics, most notably in the calibration of high-precision timekeeping devices. The measurement of the element's electron transitions provides a highly stable and reproducible frequency standard, forming the basis for atomic clocks and international time standards.