In chemistry, the molecule is the smallest indivisible portion of a pure compound that retains a set of unique chemical and physical properties. A molecule consists of two or more atoms bonded together. One might wish to view molecules analogically; as a word: the word "can" could be an example of a molecule; if "e" is added, one obtains "cane" -- in a similar fashion, H20 is the water molecule; whereas, (CH20)3 is a form of sugar molecule.

Figure 1, 3D (left and center) and 2D (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres, white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.

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 giant crystal lattices, repetitive molecules with atomic bonding (usually ionic bonding) connecting the entire structure.

A property of molecules is the integer ratio of the elements that constitute the compound, the empirical formula. 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.