Chapter 9: Molecular Geometry and Bonding Theories

The Valence-Shell Electron-Pair Repulsion model serves as the foundation for understanding molecular shape, operating on the principle that electron domains around a central atom repel one another and adopt positions that maximize distance. Students learn to categorize electron domains, differentiate between electron-domain geometry and the actual molecular geometry, and calculate expected bond angles for fundamental shapes including linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral arrangements. The treatment examines how lone pairs and multiple bonds distort idealized geometries and alter bond angles relative to theoretical predictions. Molecular polarity emerges as a consequence of shape and electronegativity differences, with asymmetrical geometries producing polar molecules while symmetric arrangements may result in nonpolar compounds. Valence-bond theory presents covalent bonding as the overlapping of atomic orbitals between atoms, introducing hybridization as a conceptual framework that explains why atoms adopt specific geometries by mixing s, p, and d orbitals into equivalent hybrid orbitals. The chapter explores how sigma bonds form from direct orbital overlap along internuclear axes while pi bonds result from parallel orbital interaction. Molecular orbital theory advances bonding understanding by treating molecules as unified systems where atomic orbitals combine to create bonding orbitals that stabilize electron placement and antibonding orbitals that destabilize it. Bond order calculations derived from molecular orbital electron configurations predict bond strength and stability, while the presence of unpaired electrons in antibonding orbitals explains magnetic behavior. Real applications demonstrate how these theories account for properties of carbon dioxide, ammonia, boron trifluoride, and coordination compounds formed by transition metals. Together, these frameworks provide predictive power for understanding why molecules behave as they do chemically and physically.