2.2.15
HL
ONLY
Sigma & Pi Bonds
Head-on vs sideways overlap.
Sigma (σ) Bond
- Head-on overlap of orbitals.
- Electron density concentrated between the two nuclei.
- All single bonds are sigma bonds.
- Free rotation possible.
Pi (π) Bond
- Sideways overlap of p-orbitals.
- Electron density above and below the bond axis.
- Found in double and triple bonds.
- Restricts rotation.
Bond Composition
| Bond Type | σ Bonds | π Bonds | Total |
|---|---|---|---|
| Single | 1 | 0 | 1 |
| Double | 1 | 1 | 2 |
| Triple | 1 | 2 | 3 |
Counting σ and π in Molecules
In ethene (C₂H₄): There are 4 C–H σ bonds + 1 C–C σ bond + 1 C=C π bond = 5σ + 1π.
In ethyne (C₂H₂): There are 2 C–H σ bonds + 1 C–C σ bond + 2 C≡C π bonds = 3σ + 2π.