Chemistry aces apply the criss-cross method for finding the formulae of binary compounds. Using aluminum chloride and magnesium oxide as examples, they are guided step-by-step through the process. This is a concise demonstration of this essential skill in chemistry. It is a handy tool for you to use when you teach your chemistry class how to find a formula and name for a polyatomic ion. Note that there are some irrelevant slides that you can hide and that the links are not working.