To figure out the ratios of these compounds, it is important to remember that the charge of these compounds must be <em>neutral</em>.
So in order to make them neutral, you must have specific ratios:
; This is true because they both have a charge of magnitude of 1.
; We need 3 chlorine atoms because we need to balance out the charge from the 3+ charge of aluminum - therefore since chlorine has a 1- charge, we need 3 atoms.
; The charges of the magnesium (2+) are balanced with the oxygen charge (2-).
; This is correct because if charges are like this, you must find the least common factor in order to know the ratio. The LCF is 6, therefore, for the atom with a 3+ charge, you need 2 of them, and for the atom with a 2- charge, you need 3 of them. This keeps the charge neutral.
So in order to make them neutral, you must have specific ratios: