7.6 Molecular Structure and Polarity

VSEPR theory predicts molecular shapes by minimizing electron pair repulsion. It considers bonding and nonbonding electron pairs, determining geometries like linear, trigonal planar, and tetrahedral. These shapes influence molecular properties and reactivity.

Bond polarity depends on electronegativity differences between atoms. Nonpolar bonds form between similar atoms, while polar bonds have uneven electron distribution. Molecular polarity combines bond polarity and geometry, affecting a molecule's behavior in reactions and solutions.

Molecular Geometry and Polarity

Molecular structures using VSEPR theory

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• VSEPR theory predicts molecular geometry by minimizing electron pair repulsion
  • Electron pairs can be bonding (shared between atoms) or nonbonding (lone pairs on a single atom)
  • Electron pairs arrange to minimize repulsion, determining the overall molecular shape (H2O, NH3)
• Electron pair geometries depend on the number of electron pairs around the central atom
  • Linear geometry occurs with 2 electron pairs (CO2, BeH2)
  • Trigonal planar geometry occurs with 3 electron pairs (BF3, SO3)
  • Tetrahedral geometry occurs with 4 electron pairs (CH4, NH4+)
  • Trigonal bipyramidal geometry occurs with 5 electron pairs (PCl5, SF4)
  • Octahedral geometry occurs with 6 electron pairs (SF6, PF6-)
• Molecular geometries are determined by the arrangement of atoms, excluding lone pairs
  • Bent geometry has 2 bonding pairs and 1 lone pair (SO2, O3)
  • Trigonal pyramidal geometry has 3 bonding pairs and 1 lone pair (PH3, AsH3)
  • Octahedral geometry has 6 bonding pairs and 0 lone pairs (WF6, UF6)
• Electron domain geometry considers both bonding and nonbonding electron pairs
  • Bond angles are influenced by electron domain geometry and repulsion between electron domains

Polar vs nonpolar covalent bonds

• Covalent bonds form when atoms share electrons
• Bond polarity depends on the electronegativity difference between bonded atoms
  • Electronegativity measures an atom's ability to attract electrons in a bond (F > O > N > Cl > Br > I)
• Nonpolar covalent bonds form between atoms with equal or similar electronegativity
  • Electrons are shared equally (C-C in ethane, H-H in H2, C-H in CH4)
• Polar covalent bonds form between atoms with different electronegativities
  • Electron density is unevenly distributed, creating a dipole moment (O-H in H2O, N-H in NH3, C-O in CO2)
• Dipole moment ($\mu$) measures bond polarity
  • Calculated as the product of charge separation ($Q$) and bond length ($r$): $\mu = Qr$
  • Larger dipole moments indicate more polar bonds (HF > HCl > HBr > HI)

Molecular polarity analysis

• Molecular polarity depends on both bond polarity and molecular geometry
• Nonpolar molecules either contain only nonpolar bonds or have polar bonds arranged symmetrically, canceling out dipole moments
  • CO2 is nonpolar due to its linear geometry, despite having polar C=O bonds
  • CCl4 is nonpolar due to its tetrahedral geometry, with polar C-Cl bonds canceling out
• Polar molecules contain polar bonds arranged asymmetrically, resulting in a net dipole moment
  • H2O is polar due to its bent geometry, with polar O-H bonds not canceling out
  • NH3 is polar due to its trigonal pyramidal geometry, with polar N-H bonds not canceling out
• Determining molecular polarity involves these steps:
  1. Identify bond polarities within the molecule based on electronegativity differences
  2. Determine the molecular geometry using VSEPR theory, considering both bonding and nonbonding electron pairs
  3. Assess if the dipole moments of polar bonds cancel out based on the molecular geometry
     • If dipole moments cancel, the molecule is nonpolar
     • If dipole moments do not cancel, the molecule is polar

Lewis Structures and Valence Electrons

• Lewis structures represent the arrangement of valence electrons in molecules
• Valence electrons are the outermost electrons involved in bonding
• Drawing Lewis structures helps visualize molecular geometry and predict bond types
• Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals, influencing molecular shape and bonding