🧶Inorganic Chemistry I Unit 2 – Chemical Bonding Theories

Key Concepts and Definitions

• Chemical bonding involves the interaction between atoms to form molecules or compounds
• Bonds form when atoms share or transfer electrons to achieve a stable electronic configuration
• Covalent bonds occur when atoms share electrons, forming a stable molecule
• Ionic bonds involve the complete transfer of electrons from one atom to another, creating oppositely charged ions
• Metallic bonding occurs in metals where valence electrons are delocalized and shared among all atoms
• Bond order represents the number of chemical bonds between two atoms in a molecule
  • Calculated using the formula: $\frac{1}{2}$(number of bonding electrons - number of antibonding electrons)
• Electronegativity measures an atom's ability to attract electrons in a chemical bond
  • Atoms with higher electronegativity tend to gain electrons, while those with lower electronegativity tend to lose electrons

Historical Development of Bonding Theories

• Early theories of chemical bonding emerged in the late 19th and early 20th centuries
• Lewis dot structures, proposed by Gilbert N. Lewis in 1916, represent valence electrons as dots around atomic symbols
  • Helped visualize the sharing of electrons in covalent bonds and the transfer of electrons in ionic bonds
• The octet rule, also proposed by Lewis, states that atoms tend to gain, lose, or share electrons to achieve a stable configuration with eight valence electrons
  • Exceptions to the octet rule include molecules with odd numbers of electrons or expanded octets
• Linus Pauling's work in the 1930s and 1940s led to the development of the valence bond theory and the concept of hybridization
• Molecular orbital theory, developed in the 1920s and 1930s by Friedrich Hund and Robert Mulliken, treats electrons as occupying orbitals that extend over the entire molecule

Valence Bond Theory

• Valence bond theory describes the formation of chemical bonds through the overlap of atomic orbitals
• Hybridization is a key concept in valence bond theory, explaining the mixing of atomic orbitals to form new hybrid orbitals
  • sp hybridization involves the mixing of one s and one p orbital, resulting in two linear hybrid orbitals (BeCl2)
  • sp2 hybridization involves the mixing of one s and two p orbitals, resulting in three trigonal planar hybrid orbitals (BF3)
  • sp3 hybridization involves the mixing of one s and three p orbitals, resulting in four tetrahedral hybrid orbitals (CH4)
• Sigma (σ) bonds form when hybrid orbitals overlap head-on, creating a strong, localized bond
• Pi (π) bonds form when unhybridized p orbitals overlap laterally, creating a weaker, delocalized bond
• Resonance structures represent the delocalization of electrons across multiple atoms in a molecule (benzene)

Molecular Orbital Theory

• Molecular orbital theory describes the behavior of electrons in a molecule using molecular orbitals that extend over the entire molecule
• Molecular orbitals are formed by the linear combination of atomic orbitals (LCAO)
  • Bonding molecular orbitals have lower energy and increased electron density between the nuclei, stabilizing the molecule
  • Antibonding molecular orbitals have higher energy and decreased electron density between the nuclei, destabilizing the molecule
• The energy level diagram shows the relative energies of molecular orbitals and helps determine the electronic configuration of a molecule
• Molecular orbital theory can explain the paramagnetic behavior of oxygen (O2) due to its unpaired electrons in antibonding orbitals
• Molecular orbital theory is particularly useful for describing the bonding in compounds with delocalized electrons, such as aromatic systems (naphthalene) and transition metal complexes (ferrocene)

Ionic and Covalent Bonding

• Ionic bonding occurs when there is a large difference in electronegativity between atoms, leading to the complete transfer of electrons
  • Ionic compounds form lattice structures held together by electrostatic attractions between oppositely charged ions (NaCl)
• Covalent bonding occurs when there is a small difference in electronegativity between atoms, leading to the sharing of electrons
  • Nonpolar covalent bonds form when electrons are shared equally between atoms with similar electronegativities (H2)
  • Polar covalent bonds form when electrons are shared unequally due to differences in electronegativity (HCl)
• The percent ionic character of a bond can be calculated using the difference in electronegativity between the atoms
• Coordinate covalent bonds, or dative bonds, form when one atom donates both electrons to the shared pair (NH4+)

Intermolecular Forces

• Intermolecular forces are attractions between molecules that influence physical properties such as boiling point and solubility
• Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule attracts the negative end of another (acetone)
• Hydrogen bonding is a strong dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (water)
  • Hydrogen bonding is responsible for the unique properties of water, such as its high boiling point and surface tension
• London dispersion forces, or induced dipole-induced dipole interactions, occur between nonpolar molecules due to temporary fluctuations in electron density (Ar)
  • The strength of London dispersion forces increases with the size and polarizability of the molecules
• Ion-dipole interactions occur between ions and polar molecules (Na+ and H2O)

Applications in Inorganic Compounds

• Bonding theories help explain the structure and properties of inorganic compounds
• Coordination compounds, or metal complexes, involve a central metal atom or ion bonded to ligands
  • The bonding in coordination compounds can be described using valence bond theory (hybridization) and molecular orbital theory (crystal field theory)
• Solid-state structures, such as perovskites (CaTiO3) and spinels (MgAl2O4), are influenced by the bonding and packing of ions
• Bonding in organometallic compounds, which contain metal-carbon bonds, can be described using the 18-electron rule and the isolobal principle
• Bonding theories are essential for understanding the reactivity and catalytic properties of inorganic compounds (Ziegler-Natta catalysts)

Experimental Techniques and Analysis

• Various experimental techniques are used to investigate chemical bonding and structure
• X-ray crystallography determines the arrangement of atoms in a crystal by analyzing the diffraction pattern of X-rays
  • Provides information on bond lengths, angles, and the overall molecular geometry
• Spectroscopic methods, such as infrared (IR) and Raman spectroscopy, probe the vibrational modes of molecules
  • IR spectroscopy is sensitive to changes in dipole moment and can identify functional groups (C=O stretching)
  • Raman spectroscopy is sensitive to changes in polarizability and can provide complementary information to IR
• Nuclear magnetic resonance (NMR) spectroscopy analyzes the magnetic properties of atomic nuclei to determine the local chemical environment
  • Helps identify the connectivity and spatial arrangement of atoms in a molecule
• Photoelectron spectroscopy (PES) measures the binding energies of electrons in molecules, providing insight into molecular orbital energies and electronic structure