AP Chemistry Unit 1 review: Atomic Structure and Properties
Unit 1 connects measured mass and composition to atoms, electrons and periodic properties. It is 7–9% of multiple choice. Expect mole calculations, mass-spectrum or photoelectron-spectrum evidence, and explanations of trends using nuclear attraction, shielding and distance.
Unit 1 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 7–9% of the multiple-choice section |
| Exam date | Thursday, May 6, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 7 questions in 105 minutes, worth 50%: three long questions (10 points each) and four short questions (4 points each). |
| Calculator | Both sections allow scientific or approved graphing calculators. Four-function calculators are allowed but not recommended. Bluebook includes both Desmos scientific and graphing calculators. |
| Class time | About 9–10 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 76% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 1 covers
College Board's course description splits the unit into 8 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 1.1 Moles and Molar Mass | Convert among mass, moles and particle counts with molar mass and Avogadro’s constant. |
| 1.2 Mass Spectra of Elements | Calculate average atomic mass from isotope masses and relative abundances. |
| 1.3 Elemental Composition of Pure Substances | Use elemental mass data to find percent composition and empirical formulas. |
| 1.4 Composition of Mixtures | Calculate the composition of a mixture from component masses or analytical data. |
| 1.5 Atomic Structure and Electron Configuration | Write electron configurations and relate shells and subshells to atomic structure. |
| 1.6 Photoelectron Spectroscopy | Use peak position and intensity to infer binding energies and electron populations. |
| 1.7 Periodic Trends | Explain atomic size and ionization-energy trends using electrostatic attraction and shielding. |
| 1.8 Valence Electrons and Ionic Compounds | Predict common ion charges and neutral ionic formulas from valence electrons. |
Keep the measured quantity separate from the particle model
Start with what the data actually measure. A mass spectrum and a photoelectron spectrum answer different questions.
| Evidence | Calculation or model | What it establishes |
|---|---|---|
| Sample mass | n = mass / molar mass | Moles of the substance, then particles = nNA. |
| Isotope abundances | Average mass = Σ(fraction × isotope mass) | A weighted mean, not an unweighted average. |
| Elemental masses | Convert each mass to moles; divide by the smallest | The simplest whole-number atom ratio. |
| PES peak position | Higher binding energy means harder electron removal | Relative attraction for electrons in different subshells. |
| PES relative intensity | Compare electrons represented by each subshell peak | Electron populations, not isotope abundance. |
| Across a period | Greater nuclear charge with similar core shielding | Valence electrons are generally held closer and more tightly. |
| Down a group | More occupied shells and shielding | Valence electrons are farther from the nucleus. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Reciting a periodic trend without a cause. Explain nuclear charge, shielding and electron–nucleus distance. The CED flags this gap on printed p. 27.
- Averaging isotope masses without weighting them. Convert percentages to fractions first. CED topic 1.2.
- Using mass ratios directly as atom ratios. Divide each elemental mass by its molar mass before reducing the ratio. CED topic 1.3.
- Confusing PES intensity with binding energy. Intensity represents relative electron population; horizontal position gives binding energy. CED topic 1.6.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 1.1
How many moles are in 7.50 g of a compound with molar mass 150.0 g mol⁻¹?
- A0.0200 mol
- B0.0500 mol
- C0.200 mol
- D20.0 mol
Show the answer
B. n = 7.50/150.0 = 0.0500 mol. The grams cancel, leaving moles. Multiplying mass by molar mass would give the wrong units.
Question 2 · 1.2
An element has isotopes of mass 62.0 u and 64.0 u at 30.0% and 70.0% abundance. What is its average atomic mass?
- A62.6 u
- B63.0 u
- C63.4 u
- D64.0 u
Show the answer
C. The weighted mean is 0.300(62.0) + 0.700(64.0) = 63.4 u. It lies closer to 64.0 u because that isotope is more abundant.
Question 3 · 1.3
A compound contains 3.60 g C and 0.600 g H. Using C = 12.0 and H = 1.00 g mol⁻¹, what is its empirical formula?
- ACH₂
- BCH
- CC₂H
- DCH₆
Show the answer
A. The amounts are 0.300 mol C and 0.600 mol H. Dividing by 0.300 gives C:H = 1:2, so CH₂ is the empirical formula. The mass ratio 6:1 is not an atom ratio.
Question 4 · 1.4
A 20.0 g mixture contains 6.00 g of salt A and 14.0 g of salt B. What is the mass percent of A?
- A70.0%
- B42.9%
- C23.1%
- D30.0%
Show the answer
D. Mass percent = component mass / total mixture mass × 100 = 6.00/20.0 × 100 = 30.0%. Dividing by the mass of B would describe a different ratio.
Question 5 · 1.5
A neutral atom has configuration 1s² 2s² 2p⁶ 3s² 3p³. How many valence electrons does it have?
- A3
- B5
- C10
- D15
Show the answer
B. The outer occupied shell is n = 3. Its 3s² and 3p³ electrons total 5. The atom has 15 total electrons, but only the five in the outer shell are counted here.
Question 6 · 1.7
Na and Mg have valence electrons in the third shell. Why is the first ionization energy of Mg higher?
- AMg has a smaller positive nuclear charge
- BMg has a new inner shell that removes shielding
- CMg attracts valence electrons more strongly with similar core shielding
- DMg has fewer protons but more neutron–electron attraction
Show the answer
C. Mg has one more proton while both atoms have the same core electron configuration. The stronger effective attraction holds Mg’s valence electrons more tightly. Neutron count does not provide the electrostatic attraction.
How to study Unit 1
Practice a chain of unit conversions from grams to moles to formula units or atoms. Write the unit beside every factor. For composition problems, use a small table of mass, molar mass and moles; reduce only the mole column.
Sketch a mass spectrum and a PES diagram separately and label both axes. Explain one peak position and one peak intensity in each. For a periodic comparison, write an explicit sentence about nuclear attraction and distance instead of citing the direction of an arrow on the periodic table.
Ashore's Chemistry course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
- AP Chemistry: all 9 units, exam format and scores
- AP Chemistry Unit 2: Compound Structure and Properties
- AP Chemistry Unit 3: Properties of Substances and Mixtures
- AP Chemistry Unit 4: Chemical Reactions
- AP Chemistry Unit 5: Kinetics
- AP Chemistry Unit 6: Thermochemistry
- AP Chemistry Unit 7: Equilibrium
- AP Chemistry Unit 8: Acids and Bases
- AP Chemistry Unit 9: Thermodynamics and Electrochemistry
Learn Chemistry one short lesson at a time
Free to start, no card. A short lesson for every topic in all 9 units.
Starting check
SAT Math · 1 of 5
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