According to the Encyclopædia Britannica, naturally occurring cesium has 78 neutrons. Cesium only has one stable, nonradioactive isotope, cesium-133, which contains a nucleus of 55 protons and 78 neutrons. Other, less common isotopes exist with a varying amount of neutrons.
Cesium occurs naturally as a soft silvery metal. It is highly reactive and is the most electropositive and alkaline element.
Atomic cesium has an incredibly reliable microwave spectrum. This facet makes cesium a natural choice for clocks to provide the fundamental unit of time, with a reliability of 1 second in 1.4 million years. The NIST-F1 cesium clock, in Boulder, Colorado, is roughly the size of a railroad flatcar.