Isomerism Molecules that have the same molecular formula but differ in the arrangement of their atoms are called isomers. Constitutional (or structural)isomers differ in their bonding sequence. Stereoisomers differ only in the arrangement of the atoms in space. 2013 Pearson Education,Inc. Chapter 2 25© 2013 Pearson Education, Inc. Isomerism • Molecules that have the same molecular formula but differ in the arrangement of their atoms are called isomers. • Constitutional (or structural) isomers differ in their bonding sequence. • Stereoisomers differ only in the arrangement of the atoms in space. Chapter 2 25