2.2.15 (HL)—Sigma (σ) and pi (π) bonds
- Syllabus
- First assessment 2025
- Objective
- 2.2.15
- Level
- HL
Sigma and Pi Bonds
HL only| Bond type | Orbital combination | Electron density |
|---|---|---|
| σ | Head-on overlap | Along the bond axis |
| π | Lateral p-orbital overlap | On opposite sides of the bond axis |
A multiple bond contains one sigma bond plus one or more pi bonds. Use the overlap and density location to distinguish the two.
Ethene contains five σ bonds—four C–H and one C–C—and one π bond; ethyne contains three σ and two π bonds. The σ framework fixes the bond axis, while sideways p-orbital overlap creates π density and restricts rotation about a double bond.
Identifying Sigma and Pi Bonds
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 2]
Describe how sigma ( σ ) and pi ( π ) bonds are formed.
σ bonds:
π bonds:
σ bonds:
orbital overlap/high electron density along internuclear axis/between nuclei
OR
head-on/end-to-end overlap «of atomic/hybridized orbitals»
π bonds:
«orbital» overlap/high electron density above and below internuclear/bond axis
OR
sideways overlap «of parallel p orbitals»
Marking guidance:
Award [2 max] for appropriate
diagrams.
The Covalent Model Summary
Retrieve the covalent pathway: shared pairs and bond order lead to geometry, polarity and molecular polarity; structure determines network properties, IMF behaviour and chromatography; HL representations extend to resonance, formal charge, sigma/pi bonds and hybridization.
Check the representation first, then count domains, apply geometry, identify polarity or forces, and connect the structure to the requested property or HL bonding description.