This program covers part 3 of the very important topic of the atomic theory of matter. We discuss how atoms combine together to form new compounds. Students review the law of conservation of mass and how the mass of the substances at the beginning of a reaction must equal the mass of the products of the reaction. We conclude by discussing the law of multiple proportions and how this law can predict how new compounds will form in nature. The entire lesson is taught by working example problems beginning with the easier ones and gradually progressing to the harder problems. Emphasis is placed on giving students confidence in their skills by gradual repetition so that the skills learned in this section are committed to long term memory.