AP Chemistry Unit 1: Atomic Structure and Properties

7 to 9% of the exam · 180 practice questions · 5 free-response questions

Chemistry begins with atoms and the mole, which links the mass you can weigh to the number of particles you cannot see. Unit 1 also covers mass spectra, electron configurations, photoelectron spectroscopy, and periodic trends. Expect to calculate moles and percent composition, interpret PES peaks, and explain trends such as ionization energy. Many students can recite trends but cannot explain them. Strong answers use Coulomb's law, nuclear charge, and distance from the nucleus, and they account for exceptions rather than just stating the pattern.

Practice this unit

White belt · 60 questions Regular · 60 questions Black belt · 60 questions Free response

White belt covers core terms and foundational ideas, Regular is set at exam level, and Black belt has harder multi-step questions.

Topics in this unit

1.1 Moles and Molar Mass (24 questions)
1.2 Mass Spectra of Elements (21 questions)
1.3 Elemental Composition of Pure Substances (21 questions)
1.4 Composition of Mixtures (18 questions)
1.5 Atomic Structure and Electron Configuration (24 questions)
1.6 Photoelectron Spectroscopy (24 questions)
1.7 Periodic Trends (24 questions)
1.8 Valence Electrons and Ionic Compounds (24 questions)

Free-response questions

Composition of a Hydrated Salt (Long question, 10 points)
Element J: PES and Isotopes (Long question, 10 points)
Reading a Mass Spectrum (Short question, 4 points)
Successive Ionization Energies (Short question, 4 points)
Empirical and Molecular Formulas (Short question, 4 points)

Sample questions

These are taken from the Regular set, with the answer and explanation shown.

Topic 1.2 Mass Spectra of Elements

Mass spectrum of a naturally occurring sample of magnesium

Questions in this group refer to this spectrum. Use the mass number of each isotope as its mass.

1. Based on the spectrum, which value is closest to the average atomic mass of magnesium?

A24.0 amu
B24.3 amu
C25.0 amu
D25.7 amu

Answer: B. Weighted average = (24 × 0.790) + (25 × 0.100) + (26 × 0.110) = 24.3 amu. It sits close to 24 because magnesium-24 is by far the most common isotope.

Common trap: Choosing 25.0 amu, the plain average of 24, 25 and 26. The isotopes are not equally common, so they must be weighted.

Topic 1.5 Atomic Structure and Electron Configuration

2. Lithium has the electron configuration 1s² 2s¹. Based on Coulomb’s law, why does removing a 1s electron from lithium take much more energy than removing the 2s electron?

AThe 1s electrons are closer to the nucleus and not shielded.
BThe 1s electrons are farther from the nucleus and more shielded.
CThe 1s electrons are attracted by more protons than the 2s electron.
DThe 1s subshell has more electrons than the 2s subshell.

Answer: A. Coulombic attraction increases as distance decreases. The 1s electrons are much closer to the nucleus than the 2s electron, and nothing shields them, so they are held much more tightly.

Common trap: Choosing “attracted by more protons.” All electrons in an atom feel the same nucleus. The difference is distance and shielding.

Topic 1.7 Periodic Trends

3. Potassium (Z = 19) and bromine (Z = 35) are both in Period 4. Which statement best explains why bromine has a much higher first ionization energy?

ABromine has a greater effective nuclear charge on its n = 4 electrons.
BBromine’s valence electrons are in a lower shell than potassium’s.
CBromine has more electrons, which repel each other and are held tightly.
DBromine has fewer core electrons shielding its valence electrons.

Answer: A. Both atoms have valence electrons in n = 4. Bromine has 16 more protons, and its extra core electrons (3d¹⁰) do not fully offset them. Its valence electrons feel a much stronger pull.

Common trap: Choosing “fewer core electrons.” Bromine actually has more core electrons (28) than potassium (18). Its much larger nuclear charge is what matters.

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