Lewis Structures and Molecular Geometry

Lewis Structures

• Lewis structures represent covalent compounds.
• Chemical formula examples: C2H6O or CH3CH2OH.

Electronegativity

• Electronegativity: Measures an atom's ability to attract shared electrons in a chemical bond.
• Pauling scale is commonly used.
• Fluorine is the most electronegative element (3.98).
• Predicts bond polarity.
• Electron Affinity: Energy released when an atom gains an electron in the gas phase.
• Measured in kJ/mol.
• High electron affinity means the atom attracts electrons strongly.

Lewis Structures: Drawing Rules

• Count total valence electrons from compound's formula.
• Determine central atom (least electronegative; H cannot be central) and connect to other elements with single bonds (2 e- each).
• Distribute remaining electrons on outer atoms then central atoms until octets are achieved or all electrons are used.
• Check if all atoms have octets (8 e-) or duets (2 for H); if short, move lone pairs from neighboring atoms into double/triple bonds.
• Examples: H2O, CO2, HCN

Formal Charge

• Helps determine the most likely Lewis structure.
• Formula: Formal\ Charge = (Valence\ Electrons) - (Nonbonding\ Electrons) - \frac{1}{2}(Bonding\ Electrons)
• Steps to calculate:
  • Count valence electrons.
  • Subtract nonbonding electrons.
  • Subtract half of bonding electrons.
• Stable structures minimize formal charges (especially on electronegative atoms).

Resonance

• Used for molecules that can't be represented by a single Lewis structure.
• True structure is a hybrid of all resonance structures, differing only in electron placement.
• Helps explain:
  • Stability of molecules
  • Equal bond lengths
  • Delocalized charge and reactivity
• Example: SO_4^{2-}

VSEPR Theory

• Predicts molecule shape based on electron pair repulsion around a central atom.
• Electron groups (single, double, triple bonds, or lone pairs) arrange to be as far apart as possible.

Common Shapes

• Based on electron groups and lone pairs (L.P.)
• 2 Electron Pairs, 0 L.P: Linear
• 3 Electron Pairs: 0 L.P: Trigonal Planar, 1 L.P: Bent
• 4 Electron Pairs: 0 L.P: Tetrahedral, 1 L.P: Trigonal Pyramidal, 2 L.P: Bent
• 5 Electron Pairs: 0 L.P: Trigonal Bipyramidal, 1 L.P: See-saw, 2 L.P: T-Shaped, 3 L.P: Linear
• 6 Electron Pairs: 0 L.P: Octahedral, 1 L.P: Square Pyramidal, 2 L.P: Square Planar, 3 L.P: T-Shaped