Overview
This lecture reviews key concepts from AP Chemistry topics 2.5 to 2.7 through multiple-choice practice questions, focusing on Lewis structures, formal charges, resonance, hybridization, VSEPR theory, and molecular polarity.
Lewis Structures and the Octet Rule
- A correct Lewis diagram must have the right total number of valence electrons and no octet rule violations.
- For polyatomic ions, adjust electron count for the ion’s charge (add for negative, subtract for positive).
- C3H6: Correct Lewis structure uses 18 valence electrons, no octet violations (Answer: D).
- NO2⁺: Correct Lewis structure uses 16 valence electrons, no octet violations (Answer: B).
Formal Charges and Stability
- Formal charge = valence electrons − (dots + bonds).
- The most stable Lewis structure has formal charges closest to zero.
- Best structure for a molecule distributes formal charges to minimize non-zero values.
- If a negative formal charge exists, it should be on the most electronegative atom.
Resonance and Bond Order
- Resonance occurs when multiple equivalent Lewis structures are possible (same atom connectivity, different bond/lone pair arrangements).
- Actual molecule is a hybrid (average) of resonance structures.
- Examples: Ozone (O₃) and acetate ion (C₂H₃O₂⁻) have resonance, resulting in equal bond lengths.
- Ozone bond order: 1.5 for each O–O bond.
VSEPR Theory, Geometry, and Bond Angles
- VSEPR (Valence Shell Electron Pair Repulsion) predicts molecular geometry from electron domains.
- Lone pairs on central atoms decrease bond angles due to higher electron repulsion.
- Bond angles: linear (180°), trigonal planar (120°), tetrahedral (109.5°), less if lone pairs are present.
- Molecule with the smallest bond angle: one with most lone pairs (e.g., NF₃ <109.5°).
Molecular Polarity
- A molecule is non-polar if bond dipoles cancel due to symmetry (e.g., BF₃).
- If bond dipoles do not cancel out (e.g., NF₃, due to lone pairs and asymmetry), the molecule is polar.
Hybridization
- Hybridization type is determined by number of electron domains:
- 2 domains = sp
- 3 domains = sp²
- 4 domains = sp³
- Each single, double, or triple bond, or lone pair, counts as one domain.
- Example: C₂H₂ (ethyne) carbon atoms are sp; after reaction to C₂H₄ (ethene), carbons are sp².
Key Terms & Definitions
- Valence Electrons — Electrons in the outermost shell used for bonding.
- Lewis Structure — Diagram showing bonds and lone pairs in a molecule.
- Octet Rule — Atoms tend to have 8 electrons in their valence shell.
- Formal Charge — Hypothetical charge on an atom in a Lewis structure.
- Resonance — Multiple Lewis structures with same atom arrangement, different electron arrangement.
- Bond Order — Average number of bonds between two atoms (can be fractional in resonance).
- VSEPR Theory — Model to predict molecular shape based on electron repulsion.
- Hybridization — Mixing of atomic orbitals (sp, sp², sp³) to explain bonding.
Action Items / Next Steps
- Review the linked packet for practice problems.
- Practice drawing and evaluating Lewis structures, assigning formal charges, identifying resonance, predicting geometry, and assigning hybridization.