Oxygen is a very prevalent and important element and is
necessary for sustaining life on this planet. This element is the
third most abundant in mass behind helium and hydrogen in
the universe. The diatomic form (O2) is the most common pure
form. With a boiling point at -183 °C, O2 exists as a colorless
and odorless gas at standard temperature and pressure. In the
process of cellular respiration, the highly reactive O2 is used as
the oxidant in breaking down food molecules to produce
energy. In turn, photosynthetic organisms generate O2 by using
energy from the sun and water. Other allotropes of pure oxygen
exist, including the trioxygen (O3) form known as ozone, as
well as other, less common allotropes of oxygen such as O4 and
O8. These oxygen allotropes are formed under high pressure
and low temperatures and are solid.
