What is a stereoisomer?

Stereoisomers are molecules that have the same structural and molecular formula but a different arrangement in space. An example of a common group of stereoisomers is from molecules with a carbon to carbon double bond. Groups bound to C=C cannot rotate due to the p orbitals overlapping and strengthening the bond. Rotation would weaken the bond energy so does not often occur. E.g. CH₃CHCHCH₃ can be two separate molecules according to the orientation of the CH₃ groups.