This PowerPoint lesson introduces formal charge as a tool for evaluating Lewis structures, helping Higher Level IBDP Chemistry students determine the most stable electron arrangement. Designed for first assessment 2025, it develops the analytical skills required to justify preferred structures when multiple valid Lewis formulas exist.
Formal charge is a theoretical bookkeeping method used to compare the valence electrons of a free atom with the electrons assigned to it in a Lewis structure. This lesson clearly defines formal charge, provides the calculation formula, and shows how it is applied to individual atoms within molecules and polyatomic ions. Students learn how to select the preferred structure by minimising non-zero charges, reducing charge separation, and placing negative charges on more electronegative atoms.
Step-by-step examples include CO₂, H₃O⁺, ICl₄⁻ and N₂O, illustrating how formal charge distinguishes between alternative structures that all satisfy the octet rule. The lesson also explains how coordination (dative) bonds affect charge distribution and reinforces the need to check that the sum of formal charges equals the overall molecular or ionic charge.
Importantly, limitations of formal charge are discussed, including the fact that it does not directly account for electronegativity differences or experimental evidence. Students are encouraged to combine formal charge analysis with chemical reasoning and resonance concepts.
The PowerPoint includes starter activities, worked examples, structured calculation steps, and practice questions suitable for Higher Level teaching, consolidation or revision.
File type included: PowerPoint (.pptx)
Last updated: January 2026 – calculation steps clarified, additional worked examples included, and HL scope checked against the 2025 IB Chemistry syllabus.
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