The most important thing to remember about ions is that you can only form an ion by adding or removing electrons. In that regard, the parent atom and the ion must always have the same number of protons.
Why is that the case? Because if you change the number of protons you change the identity of the element.
In your case, aluminium,
In order to form a
#13#protons, the same as the neutral atom
For the sake of argument, let's imagine what would happen if you were to add
At this point, you wouldn't be dealing with an aluminium atom anymore. The resulting atom would now be identified as a sulfur atom,
Assuming that you left the number of electrons unchanged, you would now have
#16#protons, the same as neutral sulfur atom
Theoretically, you would now have a
So remember, ions can only be formed by adding or removing electrons and never by changing the number of protons.