💏Intro to Chemistry Unit 8 – Advanced Theories of Covalent Bonding

Key Concepts

• Covalent bonding involves sharing of electrons between atoms to achieve stable electronic configurations
• Advanced bonding models provide more accurate descriptions of covalent bonding compared to simple Lewis structures
  • Valence Bond Theory (VBT) considers atomic orbitals and orbital overlap
  • Molecular Orbital Theory (MOT) treats molecular orbitals as linear combinations of atomic orbitals
• Molecular geometry is determined by the arrangement of atoms in a molecule and influences its properties
• Hybridization is the mixing of atomic orbitals to form new hybrid orbitals with specific geometries
• Bond properties, such as length and strength, are influenced by the electronic structure of the bonded atoms
• Bond energetics involve the energy changes associated with the formation and breaking of covalent bonds
• Intermolecular forces are attractive or repulsive forces between molecules, which affect physical properties

Covalent Bond Basics Recap

• Covalent bonds form when atoms share electrons to achieve a stable octet configuration (8 valence electrons)
• Lewis structures represent covalent bonds as shared electron pairs between atoms
• Single, double, and triple bonds differ in the number of shared electron pairs (1, 2, or 3 pairs, respectively)
• Nonpolar covalent bonds form between atoms with equal electronegativity, resulting in equal sharing of electrons
• Polar covalent bonds form between atoms with different electronegativities, resulting in unequal sharing of electrons
  • The more electronegative atom has a partial negative charge ($\delta-$), while the less electronegative atom has a partial positive charge ($\delta+$)
• Electronegativity trends across the periodic table influence the polarity of covalent bonds
  • Electronegativity generally increases from left to right and decreases from top to bottom in the periodic table

Advanced Bonding Models

• Valence Bond Theory (VBT) describes covalent bonding in terms of atomic orbital overlap
  • Atomic orbitals (s, p, d, f) combine to form bonding and antibonding molecular orbitals
  • Bonding orbitals have lower energy and increased electron density between the nuclei, stabilizing the molecule
  • Antibonding orbitals have higher energy and decreased electron density between the nuclei, destabilizing the molecule
• Molecular Orbital Theory (MOT) treats molecular orbitals as linear combinations of atomic orbitals (LCAO)
  • Molecular orbitals are formed by the constructive and destructive interference of atomic orbitals
  • Bonding molecular orbitals ($\sigma$ and $\pi$) have lower energy and increased electron density between the nuclei
  • Antibonding molecular orbitals ($\sigma*$ and $\pi*$) have higher energy and decreased electron density between the nuclei
• Molecular orbital diagrams represent the relative energies and electron occupancy of molecular orbitals
  • Electrons fill molecular orbitals in order of increasing energy, following the Aufbau principle and Hund's rule
• Bond order is the number of bonding electrons minus the number of antibonding electrons, divided by 2
  • Higher bond orders indicate stronger and shorter bonds

Molecular Geometry and Hybridization

• Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule
• VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular geometry based on the number of electron domains (bonding and nonbonding pairs)
  • Electron domains repel each other and arrange to minimize repulsion, resulting in specific geometries (linear, trigonal planar, tetrahedral, etc.)
• Hybridization is the mixing of atomic orbitals to form hybrid orbitals with specific geometries
  • sp hybridization results in linear geometry (180°)
  • sp2 hybridization results in trigonal planar geometry (120°)
  • sp3 hybridization results in tetrahedral geometry (109.5°)
• Hybrid orbitals form stronger, more directional bonds compared to unhybridized orbitals
• Molecular polarity depends on both bond polarity and molecular geometry
  • Nonpolar molecules have symmetric geometry and no net dipole moment (CO2, CH4)
  • Polar molecules have asymmetric geometry and a net dipole moment (H2O, NH3)

Bond Properties and Energetics

• Bond length is the average distance between the nuclei of two bonded atoms
  • Bond length decreases with increasing bond order (single > double > triple)
  • Bond length increases with increasing atomic size (C-C < C-Si < C-Ge)
• Bond strength is the energy required to break a bond (bond dissociation energy)
  • Bond strength increases with increasing bond order (single < double < triple)
  • Bond strength decreases with increasing atomic size (C-C > C-Si > C-Ge)
• Bond energetics involve the energy changes associated with the formation and breaking of covalent bonds
  • Bond formation is an exothermic process, releasing energy as the system becomes more stable
  • Bond breaking is an endothermic process, requiring energy input to overcome the bond dissociation energy
• Enthalpy of formation ($\Delta H_f$) is the energy change when a compound is formed from its constituent elements in their standard states
  • Negative $\Delta H_f$ values indicate stable compounds (exothermic formation)
  • Positive $\Delta H_f$ values indicate unstable compounds (endothermic formation)

Intermolecular Forces

• Intermolecular forces are attractive or repulsive forces between molecules
• Dispersion forces (London forces) arise from temporary dipoles induced by the motion of electrons
  • Dispersion forces are present in all molecules and increase with increasing molecular size and polarizability
• Dipole-dipole forces occur between polar molecules
  • Dipole-dipole forces are stronger than dispersion forces and depend on the magnitude of the molecular dipole moments
• Hydrogen bonding is a special case of dipole-dipole interaction involving hydrogen atoms bonded to highly electronegative atoms (N, O, F)
  • Hydrogen bonds are stronger than typical dipole-dipole forces and significantly influence the properties of substances (water, proteins, DNA)
• Ion-dipole forces occur between ions and polar molecules
  • Ion-dipole forces are stronger than dipole-dipole forces and are important in solvation and electrolyte solutions
• Intermolecular forces influence physical properties such as boiling point, melting point, and solubility
  • Stronger intermolecular forces result in higher boiling points, melting points, and lower volatility

Applications in Chemical Systems

• Covalent bonding and molecular structure play crucial roles in the properties and reactivity of chemical systems
• Organic molecules, such as hydrocarbons, alcohols, and amines, are held together by covalent bonds
  • The structure and bonding of organic molecules determine their physical and chemical properties
  • Functional groups (hydroxyl, carbonyl, amino) influence the reactivity and intermolecular interactions of organic compounds
• Biological macromolecules, such as proteins and nucleic acids, rely on covalent bonding and intermolecular forces
  • Peptide bonds between amino acids form the backbone of proteins
  • Hydrogen bonding between base pairs stabilizes the double helix structure of DNA
• Materials science exploits covalent bonding to design and synthesize new materials with desired properties
  • Carbon allotropes (diamond, graphite, graphene) have unique properties arising from their bonding and structure
  • Polymers are large molecules composed of repeating covalently-bonded subunits (monomers)
• Covalent bonding in semiconductors and solar cells enables the development of electronic devices and renewable energy technologies
  • Doping of silicon with elements like phosphorus or boron creates n-type and p-type semiconductors
  • Photovoltaic cells convert sunlight into electricity through the excitation of electrons in semiconductor materials

Practical Examples and Problem Solving

• Predicting molecular geometry using VSEPR theory
  • Example: Determine the molecular geometry of $\ce{NH3}$ (4 electron domains, tetrahedral arrangement, trigonal pyramidal geometry)
• Determining hybridization and orbital overlap in molecules
  • Example: Identify the hybridization and bonding in $\ce{C2H4}$ (sp2 hybridization, $\sigma$ and $\pi$ bonding)
• Calculating bond order and predicting bond properties
  • Example: Calculate the bond order of $\ce{N2}$ and compare its bond length and strength to $\ce{O2}$
• Drawing molecular orbital diagrams and determining electron configuration
  • Example: Construct the molecular orbital diagram for $\ce{HCl}$ and determine its electron configuration
• Identifying intermolecular forces in chemical systems
  • Example: Explain the differences in boiling points between ethanol and dimethyl ether based on their intermolecular forces
• Applying concepts of covalent bonding and molecular structure to real-world problems
  • Example: Discuss the role of hydrogen bonding in the secondary structure of proteins and its importance in protein folding and stability