Why is aluminium positioned in group 3 of the periodic table?

Firstly I would explain how the group numbers in the periodic table directly correlates to the number of electrons in the outer shell (valence electrons). As aluminium is said to be in group 3, this means it must have 3 electrons. To demonstrate why aluminium has 3 outer shell electrons, I would show the pupil how from the periodic table you can see aluminium has 13 electrons in total (from its atomic number). From this we would draw the Bohr model for aluminium, showing which shell each of the 13 electrons inhabits. It would then be clear, following the 2,8,8 prediction, that the 13 electrons in alluvium occupy the first three shells like so : 2,8,3. From this it would be reiterated that 3 electrons are in the outer most shell, hence the atom resides in group 3.