The minimum energy required to ionise an atom is equal to the energy to excite an electron in the atom from its ground state corresponding to , to its ionisation level corresponding to . If the amount of energy given to atom is equal to the ionisation energy, then the electron is just able to become a free electron and the atom is about to be ionised.

When this occurs, one outermost electron from the atom is just to be removed from the atom. Much higher energy is necessary to free an innermost electron from the atom because this will occur only after the outermost electrons are all removed.