Chapter 10: Modelling Bonding in Molecules

Students learn that no single model perfectly describes all molecular phenomena, so chemists strategically employ different frameworks depending on what needs explanation. The chapter begins with Lewis structures, which represent valence electron distribution through simple dot-and-line drawings and apply the octet rule to predict stable electron configurations, while acknowledging important exceptions such as electron-deficient molecules, expanded octets in period 3 and higher elements, and free radicals with unpaired electrons. When a single Lewis structure cannot fully capture a molecule's properties, the concept of resonance explains how multiple equivalent structures average into a resonance hybrid, with formal charge calculations determining which contributors dominate the actual bonding. The valence bond theory approach explains molecular geometry through orbital overlap, distinguishing between sigma bonds with cylindrical symmetry and pi bonds formed from sideways p orbital interactions, and introducing hybridization to reconcile observed three-dimensional shapes like methane's tetrahedral geometry with atomic orbital theory. The VSEPR model predicts molecular shapes by recognizing that electron-dense regions repel each other and position themselves as far apart as possible, with lone pairs occupying more space than bonding pairs and thereby reducing bond angles from ideal values. Molecular orbital theory provides the most sophisticated framework by treating electrons as delocalized across the entire molecule rather than localized between atom pairs, generating bonding orbitals at lower energy and antibonding orbitals at higher energy through mathematical combinations of atomic orbitals. The frontier orbital concept of HOMO and LUMO explains electronic transitions and chemical reactivity, while molecular orbital theory successfully predicts phenomena that simpler models cannot, including the paramagnetism of oxygen molecules and the distinct electron binding energies revealed by photoelectron spectroscopy.