Stereoisomers have the same structural formula but a different arrangement of atoms in space.E-Z isomerism is a type of stereoisomerism found in some alkenes. It happens because there is restricted rotation about the C=C double bond, as the pi bond would have to break.For E-Z isomers to exist, each carbon of the C=C must have two different groups attached. But-2-ene has E-Z isomers, but but-1-ene does not because one of its double-bond carbons has two hydrogen atoms.The Cahn-Ingold-Prelog (CIP) rules are used to name the isomers. On each carbon of the C=C, the attached atom with the higher atomic number has the higher priority.If the two higher-priority groups are on the same side of the double bond, it is the Z isomer (from zusammen, together). If they are on opposite sides, it is the E isomer (from entgegen, opposite). For example, in Z-but-2-ene the two CH3 groups are on the same side.