Chapter 11: Theories of Covalent Bonding

VB theory introduces the concept of orbital hybridization, where atomic orbitals mix to form new hybrid orbitals (sp, sp², sp³, sp³d, sp³d²) that align with observed molecular geometries such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. Students learn how hybridization explains identical bond strengths in molecules like methane and beryllium chloride, and how lone pairs occupy hybrid orbitals in bent and pyramidal structures. Section 11.2 focuses on the modes of orbital overlap, distinguishing between sigma (σ) bonds formed by end-to-end overlap and pi (π) bonds formed by side-to-side overlap. Examples from ethane, ethylene, and acetylene illustrate how single, double, and triple bonds differ in strength, electron density, and rotational freedom, with π bonds restricting rotation and leading to cis-trans isomerism. Section 11.3 introduces molecular orbital (MO) theory, which treats electrons as delocalized across the entire molecule. Students learn how atomic orbitals combine to form bonding and antibonding molecular orbitals, how MO diagrams are constructed for diatomic molecules, and how bond order is calculated to predict molecular stability. MO theory explains phenomena not addressed by VB theory, such as the paramagnetism of O₂, the unusual stability of N₂, and the instability of Be₂. The chapter also explores heteronuclear diatomics like HF and NO, showing how orbital energy differences and electronegativity affect molecular orbitals. Finally, MO theory is extended to polyatomic molecules such as benzene and ozone, eliminating the need for resonance structures by modeling delocalized π bonding. Together, VB theory and MO theory provide complementary insights: one focusing on localized orbital overlap, the other on delocalized molecular orbitals. By the end of the chapter, students can analyze bonding strength, hybridization, orbital overlap, bond order, magnetism, and delocalization to explain the structure and properties of molecules.