Molecules with the same molecular formula but a different spatial arrangement are classified as stereoisomers. These can be either enantiomers or diastereomers. Two molecules are called enantiomers if they are mirror images. They are diastereomers if no plane of reflection exists. Diastereomers can have different physical properties and reactivities. In this Demonstration, the molecules considered have two chiral centers (), thus the maximum number of stereoisomers is equal to 4 (), or 3 if the two chiral centers are identical. A meso form has two chiral centers but is superposable with its mirror image and thus optically inactive.