AP Chemistry Unit 1: Atomic Structure and Properties
Moles, mass spectra, electron configuration and periodic trends.
Unit overview
Atomic structure underpins the whole course. Moles connect the particle scale to the lab: one mole contains particles, and molar mass converts grams to moles. Mass spectra show isotopes and their relative abundance, which give the average atomic mass on the periodic table.
Electron configurations and Coulomb's law explain periodic trends. Atomic radius decreases across a period as nuclear charge grows with the same inner shielding, and increases down a group as new shells are added. Ionisation energy and electronegativity follow the opposite pattern. Photoelectron spectroscopy shows the energy needed to remove electrons from each subshell, giving direct evidence for the shell model.
Key topics
- Moles and molar mass
- Converting between mass, moles and number of particles.
- Mass spectrometry
- Average atomic mass as an abundance-weighted mean of isotope masses.
- Electron configuration and PES
- Subshell filling order and reading photoelectron spectra.
- Periodic trends
- Radius, ionisation energy and electronegativity explained with Coulomb's law.
Worked example
Chlorine is about 75% chlorine-35 and 25% chlorine-37. Estimate its average atomic mass.
- Weighted mean: .
- .
Answer: amu
Common mistakes
- Explaining a trend with more shells or more protons alone, without connecting it to the attraction between nucleus and electrons.
- Forgetting that 4s electrons are removed before 3d electrons when forming transition-metal ions.
- Reading PES peak height as energy; height shows the number of electrons, position shows binding energy.
Exam tips
- A full trend explanation names effective nuclear charge or shielding, distance and the resulting Coulombic attraction.
- Keep units in mole calculations; dimensional analysis catches most errors.
More practice on this site
- Chemistry Core Formulas (6 flashcards)
See the AP Chemistry exam format and every unit on the course page.