NaCl represents one formula unit of this substance. The smallest unit of any ionic compound is one formula unit, while the smallest particle in one formula unit is represented by one of the smallest ions which make it up. If this is not so, then add more ions on one side or the other to make it so. Now, think about what the charges mean. Aluminum has a tendency to lose 3 electrons.
Chlorine has a tendency to gain one electron.
Thus it takes 3 chlorine atoms to gain the 3 electrons that one aluminum atom can donate. These particles will all be held together by the opposite electrostatic charges. The formula is AlCl 3 . Think of writing a formula as looking for a lowest common denominator for the two charges involved. Then divide this by the charge of each ion in order to determine the subscript for the ion.