A chemical compound is a substance created when two or more different elements are connected by chemical bonds or valence forces. Compounds and elements are both distinct substances which is one of the two basic categories of matter. The other category includes mixtures of substances.

Compounds have several defining characteristics. First, they must consist of different elements. Combinations of the same atoms form molecules, not compounds. For example, when two atoms of oxygen combine they form a molecule of oxygen, not a compound. Therefore, while every compound is a molecule, not every molecule is a compound. A compound is created through a chemical reaction that creates electrostatic bonds between its atoms. Second, when elements combine they do so in definite, unchanging ratios. A water molecule, for example, always consists of two hydrogen atoms and one oxygen atom. The proportions of individual elements in a compound are...