The term descriptive chemistry is used to describe a survey of the elements, especially the main group elements. What are their oxidation states? What types of compounds do they form (with hydrogen, oxygen, nitrogen, etc.)? What types of reactions do they undergo? What are the underlying physical and chemical mechanisms involved in their reactivity. Knowledge of specific facts of chemistry is essential for an understanding of principles and concepts. These descriptive facts are taught to illustrate and illuminate fundamental chemistry principles.