Unit 1 is where AP Chemistry begins, and it's all about connecting the stuff you can measure in the lab to the particles you can't see. You'll work with moles, electron configurations, and periodic trends. These concepts form the foundation for bonding, reactions, and everything that comes after. Get these right now and the rest of the course becomes much clearer.
🎯 What You Need to Know for the Exam
Unit 1 makes up about 7-9% of the AP Chemistry exam. Focus your energy on these priorities:
What's in this review:
You can't count particles directly in a lab. You have a beaker of powder, not a counter that tells you how many atoms are inside. That's why the mole exists. The mole is the bridge between the macroscopic world you can measure and the microscopic world of atoms and molecules.
Avogadro's number (6.022 × 10²³ particles/mol) tells you how many particles are in one mole of any substance. One mole of carbon atoms contains 6.022 × 10²³ atoms. One mole of water molecules contains 6.022 × 10²³ molecules. This number is the key to all stoichiometry in chemistry.
The molar mass of a substance (in g/mol) is numerically equal to the average mass of one particle in atomic mass units (amu). Carbon-12 has an atomic mass of exactly 12 amu, so its molar mass is exactly 12 g/mol. This relationship makes dimensional analysis work in chemistry.
Key concepts to know:
⚠ Watch out for:
The mole is abstract, and students often skip the conceptual step and jump straight to math. Pause and ask yourself: what particle am I counting? How many moles do I have? What's the molar mass? Forgetting to include all atoms in a compound when calculating molar mass is a common mistake. For example, one mole of Ca(OH)2 has a mass of 40 + 2(16 + 1) = 74 g, not just 40 + 16 + 1.
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Topic
AP Chemistry: Moles, Molar Mass & Avogadro's Number
Focus on
Mole-to-gram conversions, particle counting, molar mass calculations
📝 Quiz · 15 questions
Topic
AP Chemistry: Moles and Molar Mass
Description
Converting between grams, moles, and particles using Avogadro's number and molar mass
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A mass spectrum is a graph that shows the masses of isotopes in a sample and how abundant each one is. The x-axis is the mass (in amu), and the y-axis is the relative abundance (how many atoms of that mass are present).
Reading a mass spectrum tells you two things: the isotopes present and their relative abundances. From this data, you can calculate the average atomic mass of an element. It's a weighted average based on the percentage abundance of each isotope. If chlorine has two main isotopes (Cl-35 and Cl-37), and 76% of chlorine atoms are Cl-35 while 24% are Cl-37, the average atomic mass is (0.76 × 35) + (0.24 × 37) = 35.48 amu.
Key concepts to know:
⚠ Watch out for:
Students sometimes confuse peak height with the actual percentage. Make sure you convert the height to a fractional abundance. If one peak is twice as tall as another, the first isotope is twice as abundant. Read the axis labels carefully to know whether abundance is given as a percentage or as relative abundance.
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Topic
AP Chemistry: Mass Spectra and Isotopes
Focus on
Reading mass spectra, calculating average atomic mass, understanding isotope abundance
📝 Quiz · 10 questions
Topic
AP Chemistry: Mass Spectra of Elements
Description
Interpreting mass spectra and calculating weighted averages of isotopic masses
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A pure substance contains only one type of particle: either molecules or formula units in fixed proportions. The law of definite proportions says that the ratio of element masses in any pure sample is always the same. If you measure the mass ratio of hydrogen to oxygen in water (H2O), you'll always get 2:16, no matter where your water came from.
The empirical formula is the simplest whole-number ratio of atoms in a compound. For example, hydrogen peroxide (H2O2) has an empirical formula of HO because that's the lowest ratio.
Key concepts to know:
⚠ Watch out for:
Empirical and molecular formulas are easy to confuse. The empirical formula is always the simplest; the molecular formula might be 2x, 3x, or any multiple of the empirical formula. You need additional information like molar mass to find the molecular formula from the empirical formula.
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Topic
AP Chemistry: Empirical & Molecular Formulas
Focus on
Converting percent composition to empirical formula, relating molecular to empirical formula
📝 Quiz · 10 questions
Topic
AP Chemistry: Elemental Composition
Description
Working with percent composition and deriving empirical formulas from data
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A mixture contains two or more types of particles in variable proportions. Unlike a pure substance, a mixture's composition can change. Salt water can be salty or barely salty depending on how much salt you added. Elemental analysis is a lab technique that determines the purity of a substance and the relative amounts of elements in it by burning the sample and analyzing the products.
Key concepts to know:
⚠ Watch out for:
Don't assume all 100% of a sample is the compound you're analyzing. If the problem says the sample is 95% pure, only 95% burns to give products; the rest is inert filler or impurity. Account for this when calculating.
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🃏 Flashcards · 15 cards
Topic
AP Chemistry: Mixtures and Elemental Analysis
Focus on
Distinguishing mixtures from pure substances, combustion analysis calculations
📝 Quiz · 10 questions
Topic
AP Chemistry: Composition of Mixtures
Description
Analyzing mixture composition and using elemental analysis to determine purity
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An atom has a nucleus made of protons (positive) and neutrons (uncharged), surrounded by electrons (negative). The number of protons defines the element. The number of electrons determines the charge: neutral atoms have equal protons and electrons.
Electrons occupy shells (energy levels) and subshells (sublevels) arranged according to the Aufbau principle. You fill orbitals from lowest to highest energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on. Valence electrons are the outermost electrons; core electrons are inner electrons that don't participate in bonding. The electron configuration explains an element's position on the periodic table.
Ionization energy is the energy required to remove an electron. Coulomb's law helps you understand why: electrons close to the nucleus (small distance, high effective nuclear charge) are harder to remove than electrons far away.
Key concepts to know:
⚠ Watch out for:
The Aufbau principle can be tricky. Remember that 4s fills before 3d. This matters for transition metals. Also, the exam won't ask you to assign quantum numbers, so focus on the overall order of filling and recognizing when an orbital or subshell is full.
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Topic
AP Chemistry: Electron Configuration & Atomic Structure
Focus on
Aufbau principle, writing electron configurations, identifying valence and core electrons
📝 Quiz · 15 questions
Topic
AP Chemistry: Atomic Structure
Description
Relating atomic structure to electron configuration and periodic position
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Photoelectron spectroscopy (PES) measures the energy required to remove electrons from different subshells of an atom. The instrument shoots high-energy photons at an atom, knocking electrons loose. The energy of the ejected electron tells you how tightly that electron was bound.
A PES spectrum is a graph where the x-axis is ionization energy (or binding energy) and the y-axis is the number of electrons at that energy. Each peak represents a different subshell. The peak position (x-axis) tells you how much energy it takes to remove an electron from that subshell. The peak height (y-axis) tells you how many electrons are in that subshell.
Reading a PES spectrum and connecting it to electron configuration is a heavy theme on the AP exam. If you see a peak at high energy, those are core electrons close to the nucleus. If you see a peak at low energy, those are valence electrons far from the nucleus.
Key concepts to know:
⚠ Watch out for:
Students confuse peak height (electron count) with peak position (ionization energy). A tall peak on the left side means many electrons that are easy to remove. A short peak on the right side means few electrons that are hard to remove. Also, remember that removing the first electron from each subshell requires a different energy, so you see multiple peaks even for the same element.
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Topic
AP Chemistry: Photoelectron Spectroscopy
Focus on
Reading PES spectra, relating peak position and height to subshells
📝 Quiz · 10 questions
Topic
AP Chemistry: Photoelectron Spectroscopy Data
Description
Using PES data to identify elements and determine electron configuration
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The periodic table is organized by electron configuration. Elements in the same column have the same number of valence electrons, so they have similar chemical properties. This periodicity creates predictable trends in atomic properties.
Atomic radius increases as you go down a group (more shells) and decreases as you go across a period (more protons pulling electrons in). Ionization energy (energy to remove an electron) increases across a period and decreases down a group. It's easier to remove a valence electron from a large atom far from the nucleus than from a small atom close to the nucleus.
Electronegativity (the atom's pull on electrons in a bond) increases across a period and decreases down a group. Electron affinity (the energy released when an atom gains an electron) generally becomes more negative across a period.
All these trends come from two ideas: the distance from the nucleus and the effective nuclear charge (the actual positive charge an electron "feels" after accounting for shielding from core electrons).
Key concepts to know:
⚠ Watch out for:
The periodic table shows the general trend, but there are exceptions. For example, ionization energy sometimes dips slightly when moving from s to p or from p to d orbitals because of orbital configuration changes. The exam will test your understanding of the general trends and your ability to explain them using Coulomb's law and shielding, not memorizing exceptions.
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Topic
AP Chemistry: Periodic Trends
Focus on
Ionization energy, electronegativity, atomic radius, electron affinity and their explanations
📝 Quiz · 15 questions
Topic
AP Chemistry: Periodic Trends
Description
Predicting and explaining periodic trends using Coulomb's law and electron configuration
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Valence electrons determine whether two elements will form a chemical bond. Elements in the same group of the periodic table have the same number of valence electrons, which is why they form analogous compounds. Sodium and potassium are both in Group 1, both have one valence electron, and both form +1 ions.
Typical ionic charges follow the periodic table pattern. Group 1 metals lose one electron to form +1 ions. Group 2 metals lose two electrons to form +2 ions. Nonmetals in Group 17 gain one electron to form -1 ions. Nonmetals in Group 16 gain two electrons to form -2 ions. This predictability is powerful: once you know where an element sits, you can predict its charge in an ionic compound.
Key concepts to know:
⚠ Watch out for:
Transition metals are the exception. They don't follow the simple group-to-charge pattern. Iron can form Fe2+ or Fe3+, copper can form Cu+ or Cu2+. The exam won't expect you to memorize these charges, but you should recognize that transition metals are variable and can hold variable oxidation states.
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🃏 Flashcards · 15 cards
Topic
AP Chemistry: Valence Electrons and Ionic Charges
Focus on
Identifying valence electrons, predicting charges, writing ionic formulas
📝 Quiz · 10 questions
Topic
AP Chemistry: Ionic Compounds
Description
Connecting valence electrons to reactivity and predicting ionic compound formulas
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Build connections, not isolated facts. Don't memorize electron configurations or periodic trends as random rules. Understand that Coulomb's law and shielding explain everything. Why does ionization energy increase across a period? Because the nucleus pulls harder (more protons) and electrons feel it more (less shielding). That same logic explains atomic radius, electronegativity, and electron affinity.
Practice dimensional analysis early. The mole concept shows up in every unit after this. Make sure you can convert grams to moles to particles smoothly. If this feels shaky, spend extra time here.
Use PES spectra to test your electron configuration knowledge. If you can look at a PES spectrum and write the electron configuration without mistakes, you've mastered this concept. The reverse is equally important: given an electron configuration, sketch what the PES spectrum should look like.
Create a trends reference card. Atomic radius, ionization energy, electronegativity, electron affinity. Write them all down, note whether they increase across or down, and write one sentence explaining why using Coulomb's law. This card will be your quick reference for the exam.
You've covered all the topics in Unit 1. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're ready for Unit 2.
Review Questions: Test Yourself
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Unit 1 gives you the atomic foundation. Unit 2 builds on this by exploring how atoms bond together and what structures those bonds create. Unit 3 then takes those structures and shows you how properties of substances (like boiling point, solubility, and conductivity) depend on intermolecular forces and structure.
Check out the full AP Chemistry study plan to see how this unit connects to the rest of the course.
Other Unit Reviews:
For official AP Chemistry resources, visit apcentral.collegeboard.org.
This review is aligned with the AP Chemistry Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.