Sigma and pi bonds explain how atoms share electrons to form the shapes and properties of molecules. A single covalent bond contains one sigma bond, while double and triple bonds add pi bonds on top of that sigma bond. Knowing the difference helps predict bond strength, bond length, molecular geometry, and whether parts of a molecule can rotate freely.
These ideas are especially important in organic chemistry, where carbon compounds often contain single, double, and triple bonds.
Key Facts
- A single bond = 1 sigma bond.
- A double bond = 1 sigma bond + 1 pi bond.
- A triple bond = 1 sigma bond + 2 pi bonds.
- Sigma bonds form by end-to-end orbital overlap along the internuclear axis.
- Pi bonds form by side-by-side overlap of parallel p orbitals above and below the internuclear axis.
- Bond length trend for the same atoms: single > double > triple, while bond strength trend is single < double < triple.
Vocabulary
- Sigma bond
- A sigma bond is a covalent bond formed by direct end-to-end overlap of orbitals along the line connecting two nuclei.
- Pi bond
- A pi bond is a covalent bond formed by side-by-side overlap of parallel p orbitals with electron density above and below the internuclear axis.
- Internuclear axis
- The internuclear axis is the imaginary straight line connecting the nuclei of two bonded atoms.
- Hybrid orbital
- A hybrid orbital is an orbital formed by mixing atomic orbitals on the same atom to create bonding orbitals with specific shapes and directions.
- Restricted rotation
- Restricted rotation is the limited twisting around a bond caused by the need to keep pi orbital overlap aligned.
Common Mistakes to Avoid
- Counting a double bond as two sigma bonds, which is wrong because only one bond can overlap directly along the internuclear axis. The second bond in a double bond is a pi bond.
- Drawing pi bonds as electron density between the nuclei, which is wrong because pi electron density lies above and below the internuclear axis. The sigma bond occupies the direct space between the bonded atoms.
- Assuming single, double, and triple bonds have the same length, which is wrong because adding pi bonds increases electron sharing and pulls nuclei closer together. Triple bonds are usually shortest for the same pair of atoms.
- Rotating a double bond freely in a structural drawing, which is wrong because rotation would break the side-by-side p orbital overlap. This is why double bonds can create fixed geometric arrangements.
Practice Questions
- 1 How many sigma bonds and pi bonds are present in C2H4, ethene, with structure H2C=CH2?
- 2 In C2H2, ethyne, with structure HC≡CH, how many total sigma bonds and total pi bonds are in the molecule?
- 3 Explain why a carbon-carbon double bond resists rotation, but a carbon-carbon single bond usually rotates more freely.