Why is cesium vapor used in atomic clocks?

Cesium vapor is used in atomic clocks because the cesium-133 atom provides an exceptionally stable and precise frequency standard for timekeeping.123

The operation of a cesium clock relies on the hyperfine structure of the atom's ground state. Specifically, the second is defined by the radiation frequency associated with the transition between two specific hyperfine energy levels of the cesium-133 atom. This transition occurs at exactly 9,192,631,770 cycles per second.12453

In a cesium-beam clock, a beam of cesium atoms is manipulated by magnetic fields to isolate atoms in a particular state. When these atoms are exposed to microwave radiation at the precise resonance frequency, they undergo a quantum transition. A feedback loop detects these transitions and adjusts the microwave frequency to maintain it at this constant, natural resonance. Because this frequency is invariant and highly stable, it allows for the creation of clocks that are accurate to within one second in millions of years.5123