The ground state of Cesium, as with all alkali atoms (the ones that inhabit the first column of the Periodic Table) that have one electron beyond the filled shell of the noble gases, have two ground states. The electron's spin has two possible orientations in the magnetic field of the nucleus, and each orientation has a different amount of energy. This is known as the hyperfine splitting of the ground state.

Cesium-133 has the largest hyperfine splitting of the stable alkali elements, as it is the heaviest, with the most protons and largest nuclear spin (7/2).