My understanding of the periodic table, the transitional metals all rust because the S shells are higher energy then the D shells. So all transitional metals have 2 valence electrons. (Roughly...some electrons like to move around and give different apparent charges.) So why does aluminum corrode if it doesn't have a 2+ charge?
'Rusting' commonly refers to the corrosion (oxidation) of iron so when talking about other metals, it is better to use the term 'corrosion' or 'oxidation'. Aluminum can corrode and the fact that it has a general oxidation number of +3 doesn't really matter. Many elements which have a charge that is different from +2 can oxidize. Alkali metals for instance (which have a charge of +1) can oxidize. Lithium can form lithium oxide (Li2O), sodium can form sodium oxide (Na2O) and so on. However, aluminum is known to be quite resistant to corrosion (oxidation) because it spontaneously forms a thin (solid) oxide layer at it's surface protecting it from further oxidation whereas iron, for an example, will easily lose that thin layer (it ''peels off easily'') exposing more iron to corrosion. So since Al has a +3 charge and O has a -2 charge, you'll need 2 atoms of Al and 3 atoms of O to make an electrically neutral compound. 2 atoms of Al = +6 charge 3 atoms of O = -6 charge Hence Al2O3 which is aluminum oxide. I hope it helps.
