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 two or more atoms bonded together. Not all substances are made of molecules.
Most molecules are much too small to be seen with the naked eye, but there are exceptions. A grain of salt, or the diamond on an engagement ring, are crystals, repetitive molecules with atomic bonding (usually ionic bonding) connecting the entire structure.
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.