Chapter 2: Atomic Structure and Interatomic Bonding

The treatment begins with atomic fundamentals, covering the composition of atoms through protons, neutrons, and electrons, and introducing key quantitative measures including atomic number, atomic mass, isotopes, and Avogadro's number as the bridge between atomic and macroscopic scales. Two complementary models of atomic structure are presented: the Bohr model, which describes electrons occupying discrete, quantized orbital levels around the nucleus, and the more sophisticated wave-mechanical model that replaces fixed orbits with probability distributions characterized by principal quantum number, angular momentum quantum number, magnetic quantum number, and spin quantum number. Electron configuration and the significance of valence electrons as chemically reactive outer-shell electrons are examined alongside the periodic table's organization of elements by chemical behavior. Electronegativity as a measure of electron-attracting tendency and the classification of elements into metals, nonmetals, and transition metals provide essential context for predicting bonding type and material behavior. The chapter then transitions to interatomic bonding forces, explaining how attractive and repulsive interactions balance at an equilibrium spacing corresponding to minimum bonding energy, with these parameters controlling macroscopic properties such as melting point, elastic modulus, and thermal expansion coefficient. Primary bonding comprises three distinct mechanisms: ionic bonding, characterized by electron transfer from electropositive to electronegative atoms forming charged species held by electrostatic attraction; covalent bonding, involving shared electron pairs between atoms with specific orbital overlap geometries, including hybridized bonding states exemplified by tetrahedral bonding in diamond and planar bonding in graphite; and metallic bonding, wherein valence electrons delocalize into a mobile electron cloud surrounding ion cores, accounting for electrical conductivity and plastic deformation in metals. Secondary bonding or van der Waals interactions, including London dispersion forces, induced dipole effects, and permanent dipole hydrogen bonding, though weaker than primary bonds, significantly influence properties of molecular solids, polymers, and adhesives. The bonding tetrahedron framework illustrates how most engineering materials exhibit hybrid bonding character rather than pure bonding types, with ionic-covalent mixtures dominating ceramics, metallic bonding defining metals, covalent bonding characterizing polymers, van der Waals forces governing molecular solids, and semiconductors and intermetallic compounds displaying combined bonding contributions. This conceptual integration directly links microscopic atomic interactions to macroscopic material classifications and engineering performance.