Overview
This lecture reviews three major bonding theories—molecular orbital theory, valence bond theory, and VSEPR theory—emphasizing their application in understanding molecular structure and behavior.
Molecular Orbital Theory (MO Theory)
- MO theory describes bonding by combining atomic orbitals into molecular orbitals that span the entire molecule.
- Constructive and destructive interference of electron wave functions result in bonding and antibonding orbitals.
- Stability of a molecule can be predicted by calculating bond order from its MO diagram.
- MO theory is mainly used for simple molecules and often requires computational methods.
Valence Bond Theory and Hybridization
- Valence bond theory describes bonds as overlapping atomic orbitals, with bonds forming through constructive interference.
- Hybridization explains how atoms like carbon form multiple bonds by mixing s and p orbitals.
- sp³ hybridization (one s + three p orbitals) forms four degenerate orbitals for single bonds (tetrahedral structure).
- sp² hybridization (one s + two p orbitals) forms three degenerate orbitals for double bonds, with remaining p orbital forming a pi bond.
- sp hybridization (one s + one p orbital) occurs in triple bonds, leaving two p orbitals for two pi bonds.
- Carbon atoms in single, double, and triple bonds are sp³, sp², and sp hybridized, respectively.
- Sigma (σ) bonds result from end-to-end overlap; pi (π) bonds result from side-to-side overlap.
- Single bonds are longest and weakest; triple bonds are shortest and strongest but more reactive due to weak pi bonds.
VSEPR Theory and Molecular Geometry
- VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular shapes by maximizing separation between electron groups.
- Steric number equals the total electron groups (bonds and lone pairs) around a central atom.
- Steric number of 4: sp³ hybridization (tetrahedral electron geometry).
- Steric number of 3: sp² hybridization (trigonal planar electron geometry).
- Steric number of 2: sp hybridization (linear electron geometry).
- Lone pairs affect bond angles and change molecular geometry (e.g., methane: tetrahedral; ammonia: trigonal pyramidal; water: bent).
Molecular Polarity and Electronegativity
- Molecular polarity arises from differences in electronegativity causing uneven electron distribution (induction).
- A dipole moment is created when one atom is more electronegative, resulting in partial positive and negative charges.
- Polar bonds, like C–O or O–H, have measurable dipole moments based on electronegativity differences.
Key Terms & Definitions
- Molecular Orbital (MO) Theory — Bonding theory combining atomic orbitals into molecular orbitals for the entire molecule.
- Valence Bond Theory — Describes bonds as the overlap of atomic orbitals between atoms.
- Hybridization — Mixing atomic orbitals (s, p) to form new, equivalent hybrid orbitals for bonding.
- sp³, sp², sp Hybridization — Types of orbital mixing corresponding to tetrahedral, trigonal planar, and linear geometries, respectively.
- Sigma (σ) Bond — Bond formed by end-to-end orbital overlap.
- Pi (π) Bond — Bond formed by side-to-side parallel orbital overlap.
- VSEPR Theory — Predicts molecular geometry by maximizing distance between electron groups.
- Steric Number — The number of electron groups (bonds + lone pairs) around a central atom.
- Dipole Moment — Separation of charge creating a partial positive and negative region in a molecule.
Action Items / Next Steps
- Review examples of assigning hybridization and predicting geometry using VSEPR and steric number.
- Practice identifying molecular polarity and drawing Lewis structures.
- Read textbook sections on bonding theories, hybridization, and VSEPR for reinforcement.