Lesson 6: 2.6 Resonance and Formal Charge
Duration of Days: 2
Lesson Objective
Explain the concept of resonance and use formal charge to select the most dominant Lewis structure for a given molecule or ion.
What does a fractional bond order (like 1.5) mean when analyzing a molecule with resonance?
How does calculating formal charge (FC=Valence - Lone electrons - 0.5Bonding electrons) help us determine the actual layout of a complex molecule?
Resonance Hybrid
Delocalization
Formal Charge
Bond Order
Isomer
Dominant Structure
Learning Objective 2.6.A; Suggested Skill 6.C (Argumentation - Support a claim with evidence)
Explains that resonance structures do not flip back and forth; rather, the real molecule is a hybrid blend with delocalized electrons. Students use formal charge calculations to judge nonequivalent structures, choosing the configuration that minimizes formal charges closest to zero, placing negative charges on the most electronegative atoms.
Support: Provide a simple formula grid column for formal charge calculation to keep tracking neat: FC = Group # - (Dots + Lines)
Extension: Provide three different skeletal arrangements for the fulminate ion (CNO^-1). Have students calculate formal charges for all atoms in each layout to mathematically prove which isomer is stable.
Sample Problem: A student proposes two possible Lewis structures for the thiocyanate ion, SCN^-1. Structure 1 features a triple bond between S and C, and a single bond between C and N (S triple bond C single bond N^-1). Structure 2 features a single bond between S and C, and a triple bond between C and N (S single bond C triple bond N]^-1). Calculate the formal charges for all atoms in both structures, and identify which structure is the most dominant/likely representation. Justify your selection.