Aluminium has the atomic number of 13. This means that it has 13 protons and 13 electrons. Electrons orbit the atom in shells, the first holds 2, the second holds 8. This leaves 3 extras for the third shell (valence shell). This is why Aluminium has a 3+ charge when it becomes an ion. It loses the 3 outer electrons to leave it with a full 2nd shell of 8 electrons.