This lecture completes the first half of the semester by analyzing three functional groups in terms of the interaction of localized atomic or pairwise orbitals. Many key properties of biological polypeptides derive from the mixing of such localized orbitals that we associate with "resonance" of the amide group. The acidity of carboxylic acids and the aggregation of methyl lithium into solvated tetramers can be understood in analogous terms. More amazing than the panoply of modern experimental and theoretical tools is that their results would not have surprised traditional organic chemists who already had developed an understanding of organic structure with much cruder tools. The next quarter of the semester is aimed at understanding how our scientific predecessors developed the structural model and nomenclature of organic chemistry that we still use.