cesium133atom

Cesium-133 is the neutral atom of the chemical element cesium with atomic number 55 and mass number 133. It is the only stable, naturally occurring isotope of cesium and forms the basis of the SI unit of time. In nature, cesium is found in minerals and salts; the primary commercial source is the ore mineral pollucite. The nucleus contains 55 protons and 78 neutrons (Z=55, N=78), giving a mass number of 133. The ground-state electron configuration is [Xe] 6s1, resulting in a single valence electron and the characteristic high reactivity of alkali metals.

Physically, cesium is a soft, silvery-gold metal with a low melting point of about 28.5 °C and a boiling point of 671 °C. It is highly reactive, especially with water, producing cesium hydroxide and hydrogen gas; due to its reactivity, it is typically stored under mineral oil or in an inert atmosphere.

In metrology, cesium-133 is best known for its role in defining the second. Since 1967 the second has been defined as 9,192,631,770 cycles of the microwave radiation corresponding to the transition between the two hyperfine levels of the cesium-133 ground state. Modern timekeeping relies on cesium atomic clocks, with ongoing development including optical clocks that complement but do not replace cs-133 standards. Cs-133 remains the standard reference for frequency and time, and it underpins global navigation, communications, and fundamental physics experiments.