No, a pi bond itself is not shorter than a sigma bond. However, the presence of pi bonds in addition to sigma bonds (forming double or triple bonds) results in a shorter overall bond length compared to a single (sigma) bond.
Here's a breakdown:
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Sigma Bonds: These are the primary bonds formed by direct overlap of atomic orbitals. They are generally stronger individually due to this greater overlap. All single bonds are sigma bonds.
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Pi Bonds: These are formed by the sideways overlap of p-orbitals. They are weaker than sigma bonds due to less effective overlap. Pi bonds always occur in addition to a sigma bond. A double bond consists of one sigma and one pi bond; a triple bond consists of one sigma and two pi bonds.
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Bond Length and Multiple Bonds: The more bonds that exist between two atoms (sigma + pi), the stronger the overall attraction, and the shorter the distance between the atoms. Therefore:
- Single bond (sigma bond): Longest bond length.
- Double bond (sigma + pi): Intermediate bond length.
- Triple bond (sigma + two pi): Shortest bond length.
Think of it this way: the addition of pi bonds to a sigma bond pulls the atoms closer together. The higher electron density of the double or triple bond compared to a single sigma bond leads to a greater attractive force and thus a shorter bond length. As stated in the provided reference, "the more pi bonds you have, the shorter the bond length."
