cesium133atomen

Cesium-133 (133Cs) is the most abundant and chemically significant isotope of the alkali metal cesium (element 55). It has 55 protons and 78 neutrons, giving it a nominal atomic mass of 133. Its electron configuration in the neutral atom is [Xe] 6s1, with a single valence electron that drives its characteristic reactivity as a soft, highly reactive metal. Natural cesium consists essentially entirely of the 133Cs isotope; other cesium isotopes are radioactive and occur only in trace amounts or under specific production conditions.

On the nucleus, 133Cs has nuclear spin I = 7/2, leading to hyperfine structure in its electronic states. The ground state is a 6s1/2 state, and the hyperfine transition between the F=4 and F=3 levels provides a highly stable frequency reference. The defining transition frequency is 9,192,631,770 hertz, and this exact value is used to define the SI second since 1967, via cesium-133 atomic clocks.

Cesium-133 clocks underpin modern timekeeping, telecommunications, navigation, and many scientific measurements. In practice, cesium is used in microwave-frequency atomic clocks, where laser cooling and interrogation improve stability and accuracy.

Chemically and physically, cesium is a soft, highly reactive metal that melts at 28.5 °C and boils at 671 °C. It occurs in mineral deposits such as pollucite and is extracted industrially from natural brines. In the Earth's crust, 133Cs is the dominant form of the element.