Which would have a greater value for the third ionisation energy: magnesium or aluminium?
Mg2+ has an electronic configuration of 1s22s22s6, Al2+ has an electronic configuration of 1s22s22s63s1.
Shielding in Mg2+ is lower than that of Al2+ since Al2+ will start filling a new shell.
Since the electrons are further away from the nucleus the pull on the outermost electrons will be less.
The Ionisation Energy for Mg2+ will be higher than that of Al2+.