Answer
Once an atom has formed an ion with a complete octet (a complete valence shell), the energy required to remove an electron dramatically increases. This is because an octet gives stability to the atom, no more interaction between electrons is required to fill the outer shell. This is why there is a large jump between the second and third ionization energies of magnesium and the third and fourth energies of aluminum. Such elements form complete octets when two and three electrons have been removed, respectively; removing one more electron takes considerably more energy.
Work Step by Step
Once an atom has formed an ion with a complete octet (a complete valence shell), the energy required to remove an electron dramatically increases. This is because an octet gives stability to the atom, no more interaction between electrons is required to fill the outer shell. This is why there is a large jump between the second and third ionization energies of magnesium and the third and fourth energies of aluminum. Such elements form complete octets when two and three electrons have been removed, respectively; removing one more electron takes considerably more energy.
