Molecule

In chemistry, a molecule is the smallest indivisible portion of a pure compound that retains a set of unique chemical and physical properties of that compound. A molecule consists of several atoms bonded together. Not all substances are made of molecules.

One important property of a molecule is the integer ratio of the elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers.