All elements strive to obtain a noble gas configuration by either gaining or losing electrons. Since Aluminum is in the thrid column, it has 3 valance electrons and would tend to lose the 3 electrons, than to gain 5 electrons to achieve noble gas configuration. On the other hand, since Chlorine is in the seventh column, it has 7 valance electrons and would tend to gain 1 electron, than to lose all 7 electrons to achieve noble gas configuration. Based on these tendencies, Aluminum and Chlorine would combine in a ratio such that all the Aluminum and Chlorine ions will achieve their desired noble gas configuration. This ratio would be 1 Al : 3 Cl, since it takes 3 Chlorine atoms to accept the 3 electrons that 1 Aluminum atom donates.
