S2.2 Resonance Structures, HL (IBDP Chemistry)

This PowerPoint lesson introduces resonance structures and electron delocalisation, helping Higher Level IBDP Chemistry students understand why some molecules cannot be represented by a single Lewis structure. Designed specifically for first assessment 2025, it builds the conceptual depth and exam precision required for HL organic and structural chemistry.

Resonance occurs when two or more valid Lewis structures can be drawn for the same arrangement of atoms but with different distributions of electrons. The real species is not switching between these structures; instead, it exists as a resonance hybrid with delocalised electrons and intermediate bond properties. This lesson develops this definition carefully and links it directly to experimental evidence.

The PowerPoint uses ozone (O₃) as a central case study, drawing on bond energy and bond length data to show why its O–O bonds are identical and intermediate between single and double bonds. Students are guided through how resonance explains these observations, including the use of double-headed arrows, dashed-bond hybrid representations, and the concept of bond order.

Further examples include the carbonate ion (CO₃²⁻), nitrate ion (NO₃⁻) and carbonite ion (CO₂²⁻), allowing students to practise identifying resonance, counting valid structures, and calculating fractional bond orders. Common misconceptions are explicitly addressed, particularly the idea that molecules “flip” between resonance forms.

The lesson includes starter activities, clearly defined learning outcomes, worked examples, practice questions, and answers that reflect IB command terms and markscheme expectations. It is suitable for classroom teaching, guided practice, or HL revision.

File type included: PowerPoint (.pptx)
Last updated: January 2026 – explanations refined, misconceptions addressed, and HL scope checked against the 2025 syllabus.