At ground level, earth's atmosphere consists of many kinds of molecules.

A molecule is a group of atoms bound together. About 78 percent of the molecules in the atmosphere are nitrogen molecules, each made up of two nitrogen atoms. Twenty-one percent are oxygen molecules, consisting of two oxygen atoms. Most of the rest of the atmosphere at ground level consists of argon, helium, and carbon dioxide. Other gases, such as ozone, are present in relatively small amounts.

The formula for diatomic oxygen is O₂. This form of oxygen is vital to animal life. Ozone (O₃) is a molecule made up of three oxygen atoms bound together. Pure ozone is an extremely corrosive bluish gas with a pungent smell. It can be useful under controlled conditions. For instance, it is created in factories for use as a bleach, to deodorize air, to purify water, and to treat industrial waste.

Figure 5 Oxygen and ozone Many gaseous elements exist not as individual atoms but as diatomic molecules, i.e., molecules made up of two covalently bonded atoms.  O2 is an example of a diatomic molecule. Diatomic oxygen can bond with a free oxygen atom to form ozone (O3).