Final answer:
The criss-cross method is used to determine the formula of an ionic compound such as aluminum bromide, where ion charges are crossed over to become subscripts of the other ion. In the case of aluminum bromide, the correct formula is AlBr3 after applying the criss-cross method.
Step-by-step explanation:
The criss-cross method is a way to write the formula of an ionic compound, like aluminum bromide, where the absolute values of the ion charges become the subscripts of the other ion. Take, for instance, aluminum, which has a charge of 3+ (Al3+), and bromine, which has a charge of 1- (Br1-).
When we criss-cross the charges, aluminum's 3+ charge would be used as the subscript for bromine and vice versa, yielding Al1Br3 or simply AlBr3. However, if the subscripts are not in their simplest form, they must be reduced. For example, in the case of aluminum oxide, the criss-cross method first gives Al2O3, which is already in its simplest form and does not need to be reduced.