Ionization is a process in which an electron is stripped off of a particle.
The ionization energy is the amount of energy needed to strip an electron off of an atom , ion, or molecule. The illustration shows that the metals like lithium, Li, and cesium, Cs, have relatively low ionization energies. This means it takes relatively small amounts of energy to remove electrons from these atoms. Metals tend to lose electrons and form positive ions. The nonmetals like neon, Ne, fluorine, F, and oxygen, O, have relatively high ionization energies.