The energy needed to remove an outer electron from an atom is called the atom’s ionization energy. 

(a) Use Fig. 9-29 to find the ionization energy in eV of lithium (atomic number 3) on the basis that its two inner electrons completely shield 2e of the nuclear charge from the outer electron. (Begin by establishing the principal quantum number n of the outer electron.) 

(b) The observed value of this energy is 5.4 eV. If this is not the same as the answer to (a), account for the difference.