AP Chemistry Units: The Complete List
All 9 units, their official current names, exam weighting, pacing, and every topic inside each one.
AP Chemistry has 9 units, each covering a distinct block of content and carrying its own weighting on the multiple-choice section of the exam — from 7% up to 22% for the heaviest unit. Several study guides still describe these units using names from before the Fall 2024 revision; the table below uses the current official names, verified directly against the current AP Chemistry Course and Exam Description.
All 9 Units at a Glance
| Unit | Official Name | Exam Weighting | Typical Pacing |
|---|---|---|---|
| 1 | Atomic Structure and Properties | 7–9% | ~9–10 class periods |
| 2 | Compound Structure and Properties | 7–9% | ~12–13 class periods |
| 3 | Properties of Substances and Mixtures | 18–22% | ~14–15 class periods |
| 4 | Chemical Reactions | 7–9% | ~14–15 class periods |
| 5 | Kinetics | 7–9% | ~13–14 class periods |
| 6 | Thermochemistry | 7–9% | ~10–11 class periods |
| 7 | Equilibrium | 7–9% | ~13–15 class periods |
| 8 | Acids and Bases | 11–15% | ~14–16 class periods |
| 9 | Thermodynamics and Electrochemistry | 7–9% | ~10–13 class periods |
Pacing figures come from the CED's own suggested schedule, based on a class meeting five days a week for 45-minute periods — useful as a rough planning benchmark, not a fixed requirement, since teachers adjust pacing for their own schedules.
Two things stand out from this table. First, Unit 3 alone can carry as much weight as two or three of the lighter units combined — it's the clear priority for study time. Second, pacing doesn't track weighting perfectly: Unit 8 gets the second-most class time despite being third by exam weighting, because acid-base chemistry involves a genuinely long sequence of interlocking skills (titrations, buffers, the Henderson-Hasselbalch equation) that takes real time to build up even though the exam weighting tops out lower than Unit 3.
What's Actually In Each Unit
Names and weightings only tell you so much. Here's what each unit actually covers, topic by topic, as defined in the current CED.
Unit 1: Atomic Structure and Properties (7–9%)
Moles and Molar Mass · Mass Spectra of Elements · Elemental Composition of Pure Substances · Composition of Mixtures · Atomic Structure and Electron Configuration · Photoelectron Spectroscopy · Periodic Trends · Valence Electrons and Ionic Compounds
Unit 2: Compound Structure and Properties (7–9%)
Types of Chemical Bonds · Intramolecular Force and Potential Energy · Structure of Ionic Solids · Structure of Metals and Alloys · Lewis Diagrams · Resonance and Formal Charge · VSEPR and Hybridization
Unit 3: Properties of Substances and Mixtures (18–22%)
Intermolecular and Interparticle Forces · Properties of Solids · Solids, Liquids, and Gases · Ideal Gas Law · Kinetic Molecular Theory · Deviation from Ideal Gas Law · Solutions and Mixtures · Representations of Solutions · Separation of Solutions and Mixtures · Solubility · Spectroscopy and the Electromagnetic Spectrum · Properties of Photons · Beer-Lambert Law
This is both the heaviest-weighted unit and the one with the most individual topics — 13, roughly half again as many as most other units — which is a big part of why it carries so much exam weight.
Unit 4: Chemical Reactions (7–9%)
Introduction for Reactions · Net Ionic Equations · Representations of Reactions · Physical and Chemical Changes · Stoichiometry · Introduction to Titration · Types of Chemical Reactions · Introduction to Acid-Base Reactions · Oxidation-Reduction (Redox) Reactions
Unit 5: Kinetics (7–9%)
Reaction Rates · Introduction to Rate Law · Concentration Changes Over Time · Elementary Reactions · Collision Model · Reaction Energy Profile · Introduction to Reaction Mechanisms · Reaction Mechanism and Rate Law · Pre-Equilibrium Approximation · Multistep Reaction Energy Profile · Catalysis
Unit 6: Thermochemistry (7–9%)
Endothermic and Exothermic Processes · Energy Diagrams · Heat Transfer and Thermal Equilibrium · Heat Capacity and Calorimetry · Energy of Phase Changes · Introduction to Enthalpy of Reaction · Bond Enthalpies · Enthalpy of Formation · Hess's Law
Unit 7: Equilibrium (7–9%)
Introduction to Equilibrium · Direction of Reversible Reactions · Reaction Quotient and Equilibrium Constant · Calculating the Equilibrium Constant · Magnitude of the Equilibrium Constant · Properties of the Equilibrium Constant · Calculating Equilibrium Concentrations · Representations of Equilibrium · Introduction to Le Châtelier's Principle · Reaction Quotient and Le Châtelier's Principle · Introduction to Solubility Equilibria · Common-Ion Effect
Unit 8: Acids and Bases (11–15%)
Introduction to Acids and Bases · pH and pOH of Strong Acids and Bases · Weak Acid and Base Equilibria · Acid-Base Reactions and Buffers · Acid-Base Titrations · Molecular Structure of Acids and Bases · pH and pKa · Properties of Buffers · Henderson-Hasselbalch Equation · Buffer Capacity · pH and Solubility
Unit 9: Thermodynamics and Electrochemistry (7–9%)
Introduction to Entropy · Absolute Entropy and Entropy Change · Gibbs Free Energy and Thermodynamic Favorability · Thermodynamic and Kinetic Control · Free Energy and Equilibrium · Free Energy of Dissolution · Coupled Reactions · Galvanic (Voltaic) and Electrolytic Cells · Cell Potential and Free Energy · Cell Potential Under Nonstandard Conditions · Electrolysis and Faraday's Law
Where to Focus Your Study Time
Weighting is the most reliable signal for where studying pays off most:
- Unit 3 and Unit 8 together account for close to a third of the multiple-choice section — 18–22% and 11–15% respectively. Under-preparing for either has an outsized effect on your score compared to any of the 7–9% units.
- The seven remaining units are weighted roughly equally (7–9% each), so there's no strong case for skipping any of them in favor of another same-tier unit — they contribute similarly to your final score.
- Unit content also builds cumulatively in practice: Unit 7 (Equilibrium) leans on Unit 5's rate concepts, and Unit 8 (Acids and Bases) is essentially equilibrium applied to a specific system — falling behind early tends to compound later in the course.
Related Resources
- AP Chemistry Course and Exam Description
- AP Chemistry Reference Sheet
- AP Chemistry Periodic Table
- AP Chemistry Score Calculator
- AP Chemistry FRQ Calculator
Frequently Asked Questions
How many units are in AP Chemistry?
Nine, each with its own weighting on the multiple-choice section, ranging from 7% to 22%.
Which AP Chemistry unit is the hardest?
There's no officially published difficulty ranking. What can be said with evidence: Unit 3 and Unit 8 carry the most exam weight and the most individual topics, which is why they typically require the most study time -- that's a workload difference backed by real data, not necessarily a claim about which is conceptually "hardest."
Which AP Chemistry unit is worth the most on the exam?
Unit 3, Properties of Substances and Mixtures, at 18-22% of the multiple-choice section -- more than double most other units.
Did the AP Chemistry unit names change recently?
Yes. The current CED (Effective Fall 2024) renamed Unit 2 to "Compound Structure and Properties" (previously "Molecular and Ionic Compound Structure and Properties"), Unit 3 to "Properties of Substances and Mixtures" (previously "Intermolecular Forces and Properties"), and Unit 9 to "Thermodynamics and Electrochemistry" (previously "Applications of Thermodynamics"). Many study guides online still use the older names.
Do all 9 units count equally toward my final score?
No -- each has a different exam weighting, from 7% up to 22%, so they do not contribute equally.
Where can I find more detail on the full AP Chemistry framework?
The AP Chemistry Course and Exam Description covers the full course framework, science practices, and how units and topics fit together.
Sourced from College Board's official AP Chemistry Course and Exam Description, Effective Fall 2024. This page describes the document's real structure and current terminology; it is not a copy of it and is not affiliated with or endorsed by College Board.