A molecule is the smallest particle of a substance that exhibits the chemical properties of that substance. Molecules are groups of atoms held together by relatively strong forces called chemical bond. Each molecule of a given substance always contains the same number and kinds of atoms, and the number may range from one (Ne, for example) to many thousands (protein, for example). In a chemical reaction the chemical bonds are broken, and rearrangement of atoms takes place to form molecules of new compounds. The concept of molecule dates back to approximately 1800, when a great deal of effort was being directed toward the study of games and reactions of gases. The results of the experiments led Amadeo Avogadro to suggest that some gases consisted of groups of atoms or melecules. Molecules with the same number and kinds of atoms but different properties are called isomers. There are two general kinds; structural isomers and stereoisomers. Structural isomers are molecules that diff