The majority of chemical substances are compounds. A compound is a pure substance consisting of two (or more) elements that are chemically bonded in a fixed proportion. Unlike a mixture, in which components retain their characteristics, a compound is a new entity. Thus, for example, atoms of oxygen and hydrogen can be mixed, randomly, in any proportion. The resulting mixture will contain hydrogen molecules and oxygen molecules. Water, however, which is a compound of oxygen and hydrogen, also contains oxygen and hydrogen, but its molecules combine both elements, in a fixed proportion, as expressed by the formula H₂O.

Certain elements can combine with each other in more than one proportion. For example, carbon monoxide and carbon dioxide both contain carbon oxygen. Thus a molecule of carbon monoxide (CO) consists of a carbon atom and an oxygen atom. In carbon dioxide (CO₂), a carbon atom is...