Experiment shows that the first ionization energy of lithium is less than the first ionization energy of beryllium, but the second ionization energy of beryllium is less than the second ionization energy of lithium. Explain these observations.

Strategy

The key issue in this problem is to consider whether each ionization process removes valence or core electrons. If valence electrons are removed, consider the Z_eff of the electrons. The ionization energy to remove core electrons is always high.