Why is there a significant increase in the twelfth ionization energy for aluminum?
There is a significant increase in the twelfth ionization energy for aluminium because you are removing an electron from a closed shell.
Aluminium is in Group 13 of the Periodic Table.
Its electron configuration is
By the time you have removed 11 electrons, you are down to an electron configuration of
This is a stable, complete shell, so extra energy is required to remove an electron from it.
The tenth, eleventh, and twelfth ionization energies of aluminium are 38 473, 42 646, and 201 266 kJ/mol, respectively.
It takes almost five times as much energy to remove the twelfth electron as it did to remove the eleventh electron.