Chapter 3: Models of Structure to Explain Properties

Covalent network substances such as silicon carbide and quartz consist of atoms joined by strong directional bonds throughout an extended three-dimensional lattice, resulting in exceptional hardness and high melting points because breaking these materials requires rupturing numerous covalent connections simultaneously. Ionic substances form crystalline lattices composed of alternating cations and anions held together by electrostatic attractions; their brittleness arises from the tendency of mechanical stress to misalign the lattice so that like-charged ions repel each other and cause fracture. Metallic substances feature positively charged ions embedded within a delocalized electron cloud, enabling their characteristic malleability, ductility, and superior thermal and electrical conductivity because the mobile electron sea can flow freely and the lattice can deform without breaking metallic bonds. Molecular substances consist of discrete molecular units bound internally by strong covalent bonds but held to neighboring molecules only through weak intermolecular forces, accounting for their low melting points and soft solid states. The chapter then addresses how modern analytical instrumentation reveals molecular structure; high-resolution mass spectrometry ionizes samples and separates them by mass-to-charge ratio with extraordinary precision to determine exact molecular formulas, while fragmentation patterns from the same technique reveal connectivity by tracking the loss of specific atomic groups. Infrared spectroscopy identifies functional groups by measuring absorption at characteristic vibrational frequencies, with carbonyl groups serving as a classic example, and the fingerprint region below 1500 inverse centimeters providing a unique absorption signature for each individual compound useful in forensic applications. The chapter concludes by introducing supramolecular chemistry, an emerging field studying host-guest complexes and materials held together by noncovalent interactions that extend chemistry beyond traditional categories and unlock applications in nanotechnology and materials design.