AP Chemistry: Lewis Structures and VSEPR Theory

AP Chemistry – Lewis Structures and VSEPR Theory

AP Chemistry Study Guide: Lewis Structures & VSEPR Theory

What This Is

Lewis structures are 2D diagrams that show how atoms bond and where valence electrons are located in a molecule. VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the 3D shape of molecules based on electron pair repulsion. These concepts are essential for the AP exam because they explain molecular geometry, polarity, and reactivity—key factors in chemical behavior. For example, water (H₂O) has a bent shape due to lone pairs on oxygen, which makes it polar and gives it unique properties like high surface tension and the ability to dissolve many substances.

Key Terms & Concepts

• Lewis Structure: A diagram showing valence electrons as dots and bonds as lines between atoms. Follows the octet rule (atoms tend to gain, lose, or share electrons to get 8 valence electrons).
• Valence Electrons: Electrons in the outermost shell of an atom; determine bonding behavior.
• Octet Rule: Atoms (except H, He, Li, Be, B) tend to form bonds until they have 8 valence electrons.
• ⚠️ Exceptions to the Octet Rule:
• Incomplete octet: Be (4 e⁻), B (6 e⁻)
• Expanded octet: Elements in Period 3+ (e.g., P, S, Cl) can hold >8 e⁻ due to d-orbitals.
• Resonance Structures: Multiple valid Lewis structures for a molecule (e.g., O₃, CO₃²⁻). The real structure is a hybrid of these.
• Formal Charge: Helps determine the most stable Lewis structure.
  Formula: Formal Charge = (Valence e⁻) – (Nonbonding e⁻) – ½(Bonding e⁻)
• VSEPR Theory: Electron pairs (bonding or lone) repel each other, arranging themselves as far apart as possible to minimize repulsion.
• Electron Domain Geometry: Shape based on all electron pairs (bonding + lone pairs).
• Molecular Geometry: Shape based only on atom positions (ignores lone pairs).
• Bond Angle: Angle between two adjacent bonds. Lone pairs compress bond angles (e.g., H₂O has 104.5° instead of 109.5°).
• Polar vs. Nonpolar Molecules:
• Polar: Asymmetric electron distribution (e.g., H₂O, NH₃).
• Nonpolar: Symmetric (e.g., CO₂, CH₄).
• Dipole Moment: A measure of polarity; occurs when there’s a separation of charge.

Step-by-Step: Drawing Lewis Structures & Predicting VSEPR Shapes

1. Draw the Lewis Structure

1. Count total valence electrons (add/subtract for ions).
2. Example: CO₃²⁻ → C (4) + 3×O (6) + 2 (charge) = 24 e⁻.
3. Draw the skeletal structure (least electronegative atom in the center; H and halogens are usually terminal).
4. Place single bonds (2 e⁻ each) between atoms.
5. Distribute remaining electrons to satisfy the octet rule (start with terminal atoms).
6. Check octets—if central atom lacks an octet, form double/triple bonds.
7. Calculate formal charges (if needed) to determine the best structure.

2. Determine VSEPR Geometry

1. Count electron domains (each lone pair, single/double/triple bond = 1 domain).
2. Example: NH₃ → N has 3 bonds + 1 lone pair = 4 domains.
3. Find electron domain geometry (use table below).
4. Determine molecular geometry (ignore lone pairs).
5. Predict bond angles (lone pairs reduce angles slightly).

Common Mistakes

• Mistake: Forgetting to account for charge when counting valence electrons.
  Correction: Add 1 e⁻ for each negative charge; subtract 1 e⁻ for each positive charge.

• Mistake: Assuming all atoms follow the octet rule (e.g., drawing BF₃ with 8 e⁻ on B).
  Correction: B and Be often have incomplete octets (6 e⁻ for B, 4 e⁻ for Be).

• Mistake: Ignoring lone pairs when predicting molecular geometry (e.g., calling NH₃ "tetrahedral").
  Correction: NH₃ has trigonal pyramidal geometry (1 lone pair on N).

• Mistake: Drawing resonance structures as separate molecules (e.g., O₃ flipping between two forms).
  Correction: Resonance structures are hybrids; the real molecule is an average.

• Mistake: Confusing electron domain geometry with molecular geometry (e.g., calling H₂O "tetrahedral").
  Correction: H₂O has bent molecular geometry (2 lone pairs on O).

AP Exam Insights

• Frequently Tested:
• Drawing Lewis structures for polyatomic ions (e.g., NO₃⁻, SO₄²⁻).
• Predicting molecular geometry and polarity (e.g., "Is CH₂Cl₂ polar?").
• Resonance structures (e.g., "Draw all resonance structures for CO₃²⁻").

• Formal charge calculations to determine the best Lewis structure.

• Tricky Distinctions:

• Electron domain vs. molecular geometry (e.g., NH₃ is tetrahedral in electron domain geometry but trigonal pyramidal in molecular geometry).

• Polar bonds vs. polar molecules (e.g., CO₂ has polar bonds but is nonpolar due to symmetry).

• FRQ Tips:

• If asked to "draw the Lewis structure," show all lone pairs and formal charges (if applicable).
• For VSEPR questions, state both electron domain and molecular geometry.
• Justify polarity by discussing symmetry and bond dipoles.

Quick Check Questions

1. Multiple Choice

Which of the following molecules has a trigonal pyramidal molecular geometry? (A) BF₃ (B) NH₃ (C) CH₄ (D) CO₂

Answer: (B) NH₃ Explanation: NH₃ has 3 bonding pairs and 1 lone pair, giving it a trigonal pyramidal shape.

2. Short FRQ

Draw the Lewis structure for SO₂, including all resonance structures. Predict its molecular geometry and bond angle.

Answer:
- Lewis Structure: S is central, double-bonded to one O and single-bonded to the other (with a lone pair on S). Resonance structures show the double bond alternating between the two O atoms.
- Molecular Geometry: Bent (<120°).
- Bond Angle: ~119° (lone pair compresses the angle from 120°).

Last-Minute Cram Sheet

1. Octet rule exceptions: Be (4 e⁻), B (6 e⁻), Period 3+ (expanded octet).
2. Formal charge formula: FC = (Valence e⁻) – (Nonbonding e⁻) – ½(Bonding e⁻).
3. Resonance = hybrid, not flipping structures.
4. VSEPR: Count electron domains (bonds + lone pairs) to predict shape.
5. Lone pairs compress bond angles (e.g., H₂O = 104.5°, not 109.5°).
6. Polarity: Asymmetric = polar; symmetric = nonpolar.
7. CO₂ is linear (nonpolar); H₂O is bent (polar).
8. ⚠️ NH₃ is trigonal pyramidal, not tetrahedral!
9. ⚠️ Double/triple bonds count as 1 electron domain in VSEPR.
10. ⚠️ Always check for resonance in molecules with multiple bonds!