Aluminum is in group 13 and has a valency of 3. Chlorine is in group 17 and has a valency of 1. When a compound is formed, it reflects the valencies of each of the combining elements. This can be simplified by saying that the combining elements exchange their valencies. Cl gets 3 and All gets 1 forming a compound with the formula shown above.
Rutherford found protons
Thompson found electrons and
Bohr crewed the Bohr model the most commen model of the atom used today
This really a your opinion question but my favor is Bohr
