As mentioned before, in this lesson we will focus on a theory about atoms that will help explain the nature of elements and compounds. Before we do that, let's discuss what a theory is and how it relates to other aspects of scientific knowledge. Because theories are always tentative and open to revision, we need to consider the criteria used for judging theories to determine their value. Also in this section, we will review the classification of elements and compounds with regard to how they fit in with the other categories of materials.