Why is there a noticeable increase in the fourth ionization energy relative to the third for aluminum?
To understand why the 4th ionization level is larger, consider the electron configuration of aluminum, [Ne] 3s23p1, after aluminum has lost 3 electrons to form Al3+ it has an electron configuration identical to neon. Since the noble gas electron configurations are stable it takes a lot of energy to change the electron configuration once the aluminum atom has been removed.
