Question

which element has a higher 3rd ionization energy, Al or Mg? why?

Answer 1

electronic configuration : Al = [Ne] 3s² 3p¹

Mg: [Ne] 3s²

for removing 3rd electron, Mg required more energy than Al because 3rd electron from Mg is removed from Mg⁺² which has config. [Ne] (which is a noble gas configuration while 3rd electron from Al is removed from ion Al⁺² which has config. [Ne] 3s¹ (which after removing third elecron will get noble gas configuration).

so from Mg third electron is removed from stable noble gas configuration while from Al after removing third elctron, it gets noble gets configuration. so more energy is required to remove an electron from noble gas (i.e. it is more stable)

so 3rd Ionisation energy of Mg is greater than Al