In these chemical changes, which are commonly called chemical reactions, one or more substances, called reactants, are converted to one or more new substances, called products. The concept of chemical change allows us to classify pure substances more completely. If a pure substance can not be broken down by chemical change into simpler pure substances that substance is an element. Elements, of which there are 109 in nature, are the basic building blocks of matter. If a pure substance can be broken down by chemical change into simpler pure substances that substance is a compound.
Compounds always contain elements in fixed mass ratios.
In this course we will spend a lot of time playing with models of matter at the submicroscopic level. Moreover, they are easy to work with and manipulate. Each element is made up of many, many atoms of a given type, which are unique to that element and different from the atoms of any other element. Compounds are also comprised of molecules, but the atoms which make up a given molecule are not all the same. For example, the compound water contains water molecules, each of which contain one oxygen and two hydrogen atoms.