The foundation of every other unit. Moles and molar mass, mass spectrometry, electron configuration, photoelectron spectroscopy, and the periodic trends that explain why elements behave the way they do.
Eight topics from the College Board CED, in order.
Topic 1.1
Moles & Molar Mass
The mole as a counting unit, Avogadro's number, and converting between mass, moles, and number of particles.
Topic 1.2
Mass Spectra of Elements
How a mass spectrum reveals isotopes and their relative abundances, and how to calculate average atomic mass.
Topic 1.3
Elemental Composition of Pure Substances
Percent composition and empirical formulas determined from mass data.
Topic 1.4
Composition of Mixtures
Using analytical techniques like chromatography and spectroscopy to determine the composition of mixtures.
Topic 1.5
Atomic Structure & Electron Configuration
Protons, neutrons, and electrons; writing electron configurations and orbital diagrams using the Pauli exclusion principle and Hund's rule.
Topic 1.6
Photoelectron Spectroscopy
Using PES data — binding energy and relative electron count — to confirm electron configurations.
Topic 1.7
Periodic Trends
Atomic radius, ionization energy, and electronegativity trends explained by effective nuclear charge and shielding.
Topic 1.8
Valence Electrons & Ionic Compounds
How valence electron count predicts ion charge, and the properties of resulting ionic compounds.
About Unit 1
Unit 1 is the atomic foundation for the rest of AP Chemistry. You'll learn how chemists count atoms using the mole, how a mass spectrum reveals the isotopes that make up an element, and how electrons are arranged in orbitals, subshells, and shells. You'll also learn to read photoelectron spectroscopy (PES) data as direct experimental evidence for electron configuration.
This unit is roughly 7–9% of the AP Chem exam and takes about 9–10 class periods. Its lighter exam weight is deceiving — periodic trends and electron configuration are assumed knowledge in nearly every later unit, especially Unit 2 (bonding) and Unit 3 (intermolecular forces).
Three themes from Unit 1 run through the entire course:
Theme
Structure & properties — electron configuration determines atomic behavior
Theme
Scale & quantity — the mole connects atomic-scale particles to measurable mass
Theme
Coulomb's law — charge and distance explain periodic trends across the table