Explain why the third ionization energy is much higher than the second ionization energy for an atom of magnesium.

Answer 1

If you look at the electron configuration for Mg, you see it is [Ne]3s2. The 1st IE removes the 1st valence electron from 3s. The 2nd IE removes the 2nd valence electron from 3s. Now the configuration is stable as Ne (Mg2+), so it is really difficult to remove the next electron (3rd IE) because it is much closer to the nucleus and it is stable.