Between carbon atoms in ethene, what types of bonds are present?

• A. Two sigma bonds
• B. One sigma bond and one pi bond
• C. Two pi bonds
• D. One sigma bond only

Answer: (B)

In ethene the bond between the two carbon atoms is a double bond, made of one sigma bond and one pi bond. Each carbon is sp2 hybridized, forming three sigma bonds: two with hydrogen and one with the other carbon. The remaining unhybridized p orbital on each carbon then side-on overlaps with the other, creating a pi bond. The sigma bond runs along the line between the two nuclei, while the pi bond exists above and below that plane. This combination explains why carbon–carbon double bonds are rigid and planar. There aren’t enough unhybridized p orbitals to form two pi bonds, and there isn’t a second sigma bond along the same axis, so the correct picture is one sigma plus one pi bond.