In the lithium atom (Z = 3) two electrons are in the n = 1 orbit and the third is in the n = 2 orbit. (Only two are allowed in the n = 1 orbit because of the exclusion principle, which will be discussed in Chapter 7.) The interaction of the inner electrons with the outer one can be approximated by writing the energy of the outer electron as

where E₁ = 13.6 eV, n = 2, and Z' is the effective nuclear charge, which is less than 3 because of the screening effect of the two inner electrons. Using the measured ionization energy of 5.39 eV, calculate Z'.

E = -ZE/n