AP Chemistry Unit 2: Molecular & Ionic Bonding and Structure

TLDR

• Understand how atoms bond to lower energy and achieve stability.

• Learn how to draw and interpret Lewis structures, predict shapes using VSEPR, and identify polarity.

• Master Coulomb’s law, lattice energy, and hybridization patterns.

• ✅ Download the AP Chemistry Unit 2 Cheat Sheet (PDF)

Why This Unit Matters

Bonding is chemistry’s language. Every reaction, color change, and phase shift starts with how atoms connect.
Unit 2 builds on atomic structure to explain why some substances are brittle while others bend, and why NaCl dissolves but diamond does not.
If you can describe the bond type, you can usually predict the property — a major skill tested on the AP exam.

1. Why Atoms Bond

Atoms bond to lower potential energy and reach a stable electron configuration (often an octet).

• Ionic bonding: electrons transfer from metals → non-metals.

• Covalent bonding: electrons are shared between non-metals.

• Metallic bonding: delocalized “sea of electrons” surrounding cations.

Key Relationship – Coulomb’s Law
E = k (q₁q₂)/r
Greater charge or smaller radius → stronger attraction.

2. Ionic Bonds & Lattice Energy

Ionic solids form repeating crystal lattices held by electrostatic attraction.

• Lattice energy (U): energy released when gaseous ions form a solid.
   U ∝ (q₁ × q₂)/r

• Larger charge or smaller ions → higher melting points, stronger bonds.

• Conduct electricity when molten or in solution, not as solids.

Mnemonic: “Charge tightens, size loosens.”

3. Covalent Bonds

Atoms share electrons to fill outer shells.

• Bond order = # shared pairs → 1 < 2 < 3 (in strength).

• As bond order increases, bond length decreases and bond energy increases.

• Polarity: based on difference in electronegativity (ΔEN).
   ΔEN < 0.4 → non-polar 0.4–1.7 → polar > 1.7 → ionic.

Bond Dipole Moment: μ = q × r
Stronger dipole → more polar bond.

4. Lewis Structures & Resonance

1. Count total valence electrons.

2. Choose central atom (least EN).

3. Connect atoms with bonds; distribute remaining e⁻.

4. Adjust to minimize formal charge = valence – (nonbonding + ½ bonding).

Resonance structures show delocalized electrons (e.g., NO₃⁻, O₃).
Stable = lowest formal charges, octet satisfied.

Mnemonic: “Less Charge → More Stability.”

5. VSEPR & Molecular Geometry

Shape is determined by electron domains:

• 2 → linear (180°)

• 3 → trigonal planar (120°)

• 4 → tetrahedral (109.5°)

• 5 → trigonal bipyramidal (90°, 120°)

• 6 → octahedral (90°)

Lone pairs repel more strongly → bond angles shrink (NH₃ ≈ 107°, H₂O ≈ 104.5°).
Mnemonic: “AXE method” (A = central atom, X = bonding atoms, E = lone pairs).

Visual idea: use color-coded models to compare NH₃ vs CH₄ vs H₂O.

6. Hybridization

Atomic orbitals mix to form hybrids matching geometry:

• sp → linear

• sp² → trigonal planar

• sp³ → tetrahedral

• sp³d → trigonal bipyramidal

• sp³d² → octahedral

Mnemonic: “Domain count = letters in hybrid.”

7. Molecular Polarity

Molecules are polar if bond dipoles don’t cancel.

• CO₂ → non-polar (linear)

• H₂O → polar (bent)

• CH₄ → non-polar (tetrahedral)

Polarity predicts solubility: “like dissolves like.”
Polar → dissolve in water; Non-polar → dissolve in oils.

8. Network & Metallic Solids

• Network covalent: 3D covalent lattice (diamond, SiO₂); very high melting points.

• Metallic solids: positive ions in a sea of electrons; conductive, malleable, ductile.

Mnemonic: “Network = strong, Metal = move.”

Common Pitfalls

• Ignoring formal charge when evaluating resonance.

• Forgetting that lattice energy depends on charge magnitude more than size.

• Mixing up electron geometry (domains) vs molecular geometry (atoms).

• Assuming symmetry = non-polar — not always!

• Counting hybridization incorrectly (remember to include lone pairs).

💡 Tutor Tip

Whenever you see a bonding problem, ask three quick questions:
1️⃣ Who’s bonding (metal, non-metal, both)?
2️⃣ How are electrons shared or transferred?
3️⃣ What geometry results?
That mental checklist answers 80% of FRQs on bonding and structure.

🎯 Ready for the Next Step?

Start with our free Unit 2 Cheat Sheet and get personalized support from Ivy-League tutors. Book your free AP Chemistry strategy session today.