Resonance structures are used throughout OC1 and II to explain physical and chemical properties. In the first semester, resonance use predominates around predicting and rationalizing acid-base chemistry trends and electrophilicity, nucleophilicity, and leaving group ability. Additionally, the first chapter of the textbook discusses determining the hybridization of 140 conjugated molecules using resonance structures. In the second semester, resonance is used more extensively when classifying structures as aromatic, nonaromatic, or antiaromatic, explaining the regioselectivity and kinetics of reactions of aromatic molecules, comparing the reactivity of carboxylic acid derivatives, and describing the structure and reactivity of enolates.