Chapter 9: Covalent Bonding: Orbitals, Hybridization, Molecular Orbital Theory

Hybridization describes how atomic orbitals combine to form hybrid orbitals suited for bonding; sp³ hybridization produces four equivalent tetrahedral orbitals in methane, sp² hybridization creates three trigonal planar orbitals in ethylene, and sp hybridization generates two linear orbitals in acetylene and carbon dioxide. The model extends to expanded valence configurations in main-group elements, where d orbitals participate in hybridization, such as dsp³ hybridization in phosphorus pentachloride and d²sp³ hybridization in sulfur hexafluoride and xenon compounds. Students learn a systematic approach: construct a Lewis structure, apply valence shell electron pair repulsion theory to predict molecular shape, then identify the appropriate hybridization scheme. The chapter then shifts focus to molecular orbital theory, which treats bonding as delocalized across the entire molecule rather than between specific atom pairs. By combining atomic orbitals mathematically, bonding orbitals form with lower energy and stability, while antibonding orbitals form with higher energy and destabilizing effects. Bond order, calculated as half the difference between bonding and antibonding electrons, quantifies bonding strength and predicts stability. Applications to homonuclear diatomic molecules including hydrogen, nitrogen, oxygen, and fluorine reveal how molecular orbital diagrams predict experimental properties like bond dissociation energy, internuclear distance, and paramagnetism, notably explaining why dioxygen exhibits unpaired electrons despite electron pairing intuitions. Photoelectron spectroscopy validates these predictions by measuring ionization energies that correspond to molecular orbital energies. Heteronuclear diatomics such as nitrogen monoxide, hydrogen fluoride, and the cyanide ion demonstrate how electronegativity differences modify orbital overlap and molecular polarity. The chapter concludes by synthesizing both models to explain resonance and delocalized pi bonding in ozone, nitrate ion, and benzene, where molecular orbital contributions account for equivalent bond lengths and exceptional stability in aromatic systems that localized models alone cannot fully describe.