IB DP Chemistry - S2.2.15 Sigma and Pi Bonds- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – S2.2.15 Sigma and Pi Bonds – Study Notes – New Syllabus
IITian Academy excellent Study Notes and effective strategies will help you prepare for your IB DP Chemistry exam.
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Chemistry 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Structure 2.2.15 — Sigma (σ) and Pi (π) Bonds
Structure 2.2.15 — Sigma (σ) and Pi (π) Bonds
Sigma (σ) bonds:
Sigma (σ) bonds are formed by the head-on (axial) overlap of atomic orbitals.
The electron density is concentrated along the bond axis—the line joining the two nuclei.
Pi (π) bonds:
Pi (π) bonds are formed by the side-on (lateral) overlap of unhybridized p orbitals.
The electron density lies above and below the bond axis.
Bond Order and Bond Composition
- Single bond = 1 σ bond only
- Double bond = 1 σ bond + 1 π bond
- Triple bond = 1 σ bond + 2 π bonds
Sigma vs Pi Bonds
| Feature | Sigma (σ) Bond | Pi (π) Bond |
|---|---|---|
| Orbital overlap | Head-on | Side-on |
| Electron density location | Along bond axis | Above and below bond axis |
| Bond strength | Stronger (greater overlap) | Weaker (less overlap) |
| Rotation around bond | Allowed | Restricted |
| Presence in bonds | All bonds have 1 σ | Only in multiple bonds |
Examples (Organic & Inorganic)
- \( \text{H}_2 \): 1 σ bond (s-s overlap)
- \( \text{CH}_4 \): 4 σ bonds (sp³-s overlap)
- \( \text{C}_2\text{H}_4 \) (ethene): each C=C has 1 σ (sp²-sp²) + 1 π (p-p)
- \( \text{C}_2\text{H}_2 \) (ethyne): each C≡C has 1 σ + 2 π bonds
- \( \text{N}_2 \): Triple bond → 1 σ + 2 π bonds
- \( \text{O}_2 \): Double bond → 1 σ + 1 π bond
- \( \text{CO}_2 \): Each C=O → 1 σ + 1 π → total = 2 σ + 2 π
Example
How many sigma and pi bonds are there in an ethyne molecule \( \text{C}_2\text{H}_2 \)?
▶️ Answer/Explanation
Structure: H−C≡C−H
- C≡C triple bond → 1 σ + 2 π bonds
- Each C−H bond = 1 σ bond
Total: 3 σ bonds, 2 π bonds
Example
Determine the number of sigma and pi bonds in \( \text{CO}_2 \).
▶️ Answer/Explanation
Structure: O=C=O
- Each C=O double bond → 1 σ + 1 π
Total: 2 σ bonds, 2 π bonds