Original study example
Draw ethene, C2H4. Count the sigma and pi bonds, then explain what is different from the bonding in ethane, C2H6.
Work through the example
Ethene has four C–H sigma bonds and one C–C sigma bond, plus one pi bond. Ethane has six C–H sigma bonds and one C–C sigma bond, with no pi bond. A double bond is not two identical sigma bonds. Use a clear drawing to make the count checkable.