🟣 This is Higher Level (HL) content.
Definitions and Orbital Overlap
Covalent bonds result from the overlap of atomic orbitals. The way these orbitals overlap determines whether a sigma or pi bond is formed.
σ Sigma Bonds
Formed by the direct, head-on (axial) combination of atomic orbitals. The electron density is concentrated directly between the nuclei, along the internuclear bond axis.
Can form from the overlap of two s orbitals, two p orbitals, an s and a p orbital, or hybrid orbitals.
π Pi Bonds
Formed by the lateral (side-on) combination of adjacent, unhybridised p orbitals. The electron density is concentrated in two regions above and below the plane of the bond axis.
Only unhybridised p orbitals can form pi bonds.
Sigma vs Pi: Key Differences
| Sigma (σ) Bond | Pi (π) Bond | |
|---|---|---|
| Overlap type | Head-on (axial, end-to-end) | Lateral (side-to-side) |
| Orbitals involved | s–s, s–p, p–p, or hybrid–hybrid | Unhybridised p–p only |
| Electron density | Along the bond axis | Above & below the axis |
| Bond strength | Stronger (greater degree of overlap) | Weaker (less orbital overlap) |
| Rotation | Free rotation ✅ | Prevents rotation ❌ |
Key Properties
💪 Bond Strength
Sigma bonds are stronger than pi bonds because the direct, head-on overlap of the orbitals allows for a greater degree of orbital overlap. Pi bonds are generally weaker due to a significantly lower degree of lateral overlap.
🔄 Rotation
Molecules with only sigma bonds allow for free rotation around the bond axis. Pi bonds restrict rotation because rotating the atoms would break the parallel overlap of the p orbitals, disrupting the electron cloud. This restricted rotation gives rise to geometric (cis-trans) isomers.
Bond Composition
The first bond that forms between two atoms is always a sigma bond. Any additional bonds within a double or triple bond are pi bonds.
- Single bond = 1σ
- Double bond = 1σ + 1π
- Triple bond = 1σ + 2π
Worked Examples: Organic Molecules
📐 Ethane (C₂H₆)
1 C–C single bond + 6 C–H single bonds
Total: 7σ bonds, 0π bonds
📐 Ethene (C₂H₄)
1 C=C double bond + 4 C–H single bonds
Total: 5σ bonds, 1π bond
📐 Ethyne (C₂H₂)
1 C≡C triple bond + 2 C–H single bonds
Total: 3σ bonds (1 from C–C + 2 from C–H) and 2π bonds (both in the C≡C)
Worked Examples: Inorganic Molecules
| Molecule | Structure | σ bonds | π bonds |
|---|---|---|---|
| H₂ | H–H single bond (s–s overlap) | 1 | 0 |
| O₂ | O=O double bond | 1 | 1 |
| N₂ | N≡N triple bond | 1 | 2 |
| CO₂ | O=C=O (two double bonds) | 2 | 2 |
⚠️ Examiner Trap
Every bond has exactly one σ component: the first bond formed is always σ. Pi bonds are additional bonds only. A "triple bond" is NOT "3 bonds of equal strength."
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