AP Chemistry units: all nine, and how much each is worth
AP Chemistry has nine units, from atomic structure to electrochemistry. Unit 3 alone is worth about a fifth of the multiple-choice section, so knowing the weightings helps you plan your revision.
On this page
The nine units at a glance
The percentages are College Board's weightings for the multiple-choice section.
| Unit | Weighting |
|---|---|
| 1. Atomic Structure and Properties | 7% to 9% |
| 2. Compound Structure and Properties | 7% to 9% |
| 3. Properties of Substances and Mixtures | 18% to 22% |
| 4. Chemical Reactions | 7% to 9% |
| 5. Kinetics | 7% to 9% |
| 6. Thermochemistry | 7% to 9% |
| 7. Equilibrium | 7% to 9% |
| 8. Acids and Bases | 11% to 15% |
| 9. Thermodynamics and Electrochemistry | 7% to 9% |
What each unit covers
- Unit 1: Atomic Structure and Properties. Moles and molar mass, mass spectra, electron configuration, photoelectron spectroscopy, and periodic trends.
- Unit 2: Compound Structure and Properties. Ionic, covalent and metallic bonding, Lewis diagrams, resonance, VSEPR shapes and hybridisation.
- Unit 3: Properties of Substances and Mixtures. Intermolecular forces, solids, liquids and gases, the ideal gas law, solutions and mixtures, separation, and spectroscopy including Beer's law. The biggest unit.
- Unit 4: Chemical Reactions. Net ionic equations, stoichiometry, titration, and types of reaction: acid-base, redox and precipitation.
- Unit 5: Kinetics. Rate laws, integrated rate laws and half-life, reaction mechanisms, collision theory and catalysts.
- Unit 6: Thermochemistry. Endothermic and exothermic processes, calorimetry, enthalpy of reaction, bond enthalpies and Hess's law.
- Unit 7: Equilibrium. K and Q, calculating equilibrium concentrations, Le Châtelier's principle, and solubility equilibria (Ksp).
- Unit 8: Acids and Bases. pH and pOH, strong and weak acids and bases, buffers, titration curves and pKa.
- Unit 9: Thermodynamics and Electrochemistry. Entropy, Gibbs free energy, the link between ΔG° and K, galvanic and electrolytic cells.
The exam format
| Section | Questions | Time | Score |
|---|---|---|---|
| I: Multiple choice | 60 | 1 hour 30 minutes | 50% |
| II: Free response | 7: 3 long (10 points each) and 4 short (4 points each) | 1 hour 45 minutes | 50% |
The exam is hybrid digital: you answer the multiple-choice questions in the Bluebook app and handwrite your free-response answers. A calculator is allowed throughout, and you get a periodic table and a sheet of equations and constants. The May 2027 exam is on Thursday 6 May 2027 (afternoon session).
How to plan your review
- Start with Unit 3, then Unit 8. Together they are worth about a third of the multiple-choice section.
- Make particle-level explanations a habit. Many free-response points ask why, and the answer is about particles and the forces between them.
- Use the official free-response questions. Every released year is on our AP Chemistry past papers page, with scoring guidelines.
Common mistakes
- Spending equal time on every unit, when Unit 3 carries two to three times the weight of most others.
- Learning definitions without practising particle-level explanations.
- Leaving acids, bases and buffers until last: they appear in both sections almost every year.
Frequently asked questions
How many units are in AP Chemistry?
Nine, from Atomic Structure and Properties to Thermodynamics and Electrochemistry.
Which AP Chemistry unit is worth the most?
Unit 3, Properties of Substances and Mixtures, at 18% to 22% of the multiple-choice section, followed by Unit 8, Acids and Bases, at 11% to 15%.
Is the AP Chemistry exam changing in 2027?
AP Central lists no format change for May 2027: 60 multiple-choice questions in 90 minutes and 7 free-response questions in 105 minutes.
Key takeaways
- Nine units; Unit 3 (18% to 22%) and Unit 8 (11% to 15%) carry the most weight.
- 60 multiple-choice questions, then 3 long and 4 short free-response questions.
- Practise explaining at the particle level.
Source: College Board, AP Central course and exam pages for AP Chemistry, checked 20 October 2026.