Stereochemistry is the study of how molecules are affected by the way their atoms are arranged in space. It is also known as 3D chemistry as the word "stereo" means "three dimensional". Using stereochemistry, chemists can work out the relationships between different molecules that are made up from the same atoms. They can also study the effect on the physical or biological properties these relationships give molecules. When these relationships influence the reactivity of the molecules it is called dynamic stereochemistry. 