What is the ratio of the density of a substance to the density of a standard?

Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in scientific usage.

The answer for this is a

Answer ozone (o3) is made of three atoms of oxygen, it is a molecule;