In the gas phase, the energy difference between a cesium atom and its most stable ion is referred to as

Cesium is in the first group, so it forms an ion with a charge of + 1. The change that cesium atoms undergo when they are in the gaseous state to form gaseous ions is known as the ionization energy. The ionization energy, by definition, is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of + 1 ions from the atoms.