Why this unit matters
Structure at the bonding level determines reactivity, phase behavior, solubility, and material properties.
What you will learn
- Represent ionic and covalent compounds with Lewis structures and formal charge reasoning.
- Use VSEPR and hybridization ideas to predict molecular geometry and bond angles.
- Relate intermolecular forces to boiling point, volatility, and solubility trends.
Understand the core ideas
Bonding models explain why substances differ in melting point, conductivity, solubility, and mechanical behavior. Ionic substances are best treated as extended lattices of cations and anions rather than as individual molecular units. Covalent substances form when atoms share electron density, producing discrete molecules or network solids depending on connectivity. Metallic bonding is different again, with mobile valence electrons distributed through a lattice of positive ion cores, which explains electrical conductivity and malleability in metals. AP Chemistry expects you to choose an appropriate structural model before predicting properties, because a wrong model gives wrong conclusions. For example, treating NaCl(s) as separate neutral molecules can lead to incorrect predictions about conductivity and phase behavior. Good explanations tie observable properties to electrostatic attraction strength, lattice or molecular arrangement, and electron mobility. That approach is more reliable than memorizing isolated property lists without a structural cause. It also improves your ability to justify rankings on free-response prompts.
Lewis structures organize valence electrons so you can test octet completion, formal charge distribution, and resonance plausibility. Formal charge is a bookkeeping tool, not a measurable physical charge map, but it helps compare alternative structures using consistent rules. In many AP contexts, the preferred contributor minimizes large charge separation and places negative charge on more electronegative atoms when possible. After a valid structure is drawn, VSEPR reasoning turns electron-domain count into molecular geometry, and geometry then controls dipole cancellation and net polarity. Polarity and intermolecular forces together explain trends in boiling point, viscosity, volatility, and miscibility. These trends are comparative rather than absolute, so ranking tasks require explicit logic from structure to force type to observed property. Strong answers state assumptions, name the dominant intermolecular force, and avoid overgeneralizing from one special case to all compounds that share only one superficial feature. Consistent notation helps prevent geometry and polarity mistakes under exam time pressure.
Key terms
- formal charge
- Bookkeeping charge on an atom in a Lewis structure based on valence electron accounting.
- resonance
- Multiple valid Lewis representations for one species that differ only in electron placement.
- VSEPR
- Model that predicts geometry from repulsion between electron domains around a central atom.
- intermolecular forces
- Attractive forces between molecules, such as London dispersion, dipole-dipole, and hydrogen bonding.
Use formal charge to compare two nitrate Lewis structures
Assume NO3- with one N center and three O atoms. Compare a structure with one N=O and two N-O single bonds against a hypothetical structure with three N-O single bonds. Use formal charge formula FC = valence - (nonbonding + 0.5 x bonding electrons).
- 1) For the one-double-bond structure: N has valence 5, nonbonding 0, bonding electrons 8, so FC(N) = 5 - (0 + 4) = +1.
- 2) In that structure, double-bond O has FC = 6 - (4 + 2) = 0, and each single-bond O has FC = 6 - (6 + 1) = -1.
- 3) Sum charges: +1 + 0 + (-1) + (-1) = -1, which matches nitrate charge with limited charge separation.
- 4) For the three-single-bond structure, FC(N) = 5 - (0 + 3) = +2 and each O is -1, giving larger charge separation overall.
A common misconception
Claim: A resonance structure means the molecule flips back and forth between separate real structures.
Correction: Resonance forms are drawings for one delocalized electron distribution. The actual species is a single resonance hybrid, not a time-sharing switch between isolated structures.
Lessons in this unit
- Ionic, covalent, and metallic bondingDistinguish bonding types and resulting structural models for solids and molecules.
- Lewis structures and formal chargeBuild valid electron-dot structures and choose plausible resonance contributors.
- Molecular geometry and polarityPredict shape and net dipole from electron domains and molecular symmetry.
- Intermolecular forces and propertiesUse LDF, dipole-dipole, and hydrogen bonding to explain property differences.
Study task
Unit checkpoint
What are the electron-domain geometry, molecular geometry, and polarity of CO2?
CO2 has 2 electron domains around carbon, so electron-domain geometry is linear. Molecular geometry is linear (180 degrees), and the molecule is nonpolar because bond dipoles cancel.