Pi bonds are shorter than sigma bonds because they contribute to a greater overall bond order and increased electron density between the bonded atoms, leading to a stronger attractive force and thus a shorter bond length.
Here's a more detailed explanation:
Bond Order and Bond Length
The bond order refers to the number of chemical bonds between a pair of atoms. Single bonds have a bond order of 1, double bonds have a bond order of 2 (one sigma and one pi), and triple bonds have a bond order of 3 (one sigma and two pi). A higher bond order generally corresponds to a shorter bond length.
Sigma vs. Pi Bonds
-
Sigma (σ) Bonds: These are formed by the direct, head-on overlap of atomic orbitals. This results in electron density concentrated along the internuclear axis (the line connecting the two nuclei). Sigma bonds are generally stronger than pi bonds individually.
-
Pi (π) Bonds: These are formed by the sideways, parallel overlap of p orbitals. This results in electron density concentrated above and below the internuclear axis.
Why Pi Bonds Shorten the Overall Bond
While a sigma bond exists in both single, double and triple bonds, the addition of pi bonds to form multiple bonds increases the electron density between the atoms. This increased electron density leads to:
-
Greater Attraction: The higher concentration of negatively charged electrons between the positively charged nuclei results in a stronger electrostatic attraction.
-
Shorter Distance: To maximize this attraction, the atoms are pulled closer together, shortening the bond length. As stated in the reference, "the more overlap the shorter the bond length." The formation of a pi bond increases the orbital overlap between the two atoms involved in the bond.
Example: Carbon-Carbon Bonds
| Bond Type | Bond Order | Bond Length (pm) |
|---|---|---|
| C-C (single) | 1 | 154 |
| C=C (double) | 2 | 134 |
| C≡C (triple) | 3 | 120 |
As you can see, as the number of pi bonds increases (from single to double to triple bonds), the bond length decreases.
Conclusion
The presence of pi bonds, in addition to sigma bonds, increases the overall bond order and electron density between atoms. This leads to a stronger attractive force and a shorter bond length in double and triple bonds compared to single bonds.
