Atomic Spectra

For longest wavelength in Lyman series,

The minimum energy to ionize an atom is the energy required to excite an electron in the atom from its ground state corresponding to to its ionization level corresponding to .
If the electron is given an amount of energy equal to the ionization energy, the electron is just able to become a free electron and the atom is about to be ionized. 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 outer electrons are all removed.