Unit 1: Atomic Structure and Properties
AP Chemistry: 93 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 1 builds the counting and structure toolkit the rest of the course uses. You quantify matter with moles and molar mass, read elemental composition from mass spectra, and connect electron configuration to periodic trends and photoelectron spectra.
The mole and molar mass
One mole is Avogadro's number (6.022 × 10²³) of particles. Molar mass is the mass of one mole in g/mol — numerically equal to the average atomic mass on the periodic table. Use it as the conversion hub: grams ↔ moles ↔ particles ↔ formula units.
Mass spectrometry and isotopic composition
A mass spectrometer ionizes atoms and separates the ions by mass-to-charge ratio. Each isotope produces a peak at its mass with a height proportional to its abundance. The weighted average of the peaks gives average atomic mass. Relative heights, not positions, encode abundance.
Electron configuration and PES
Electrons fill subshells by energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, … (aufbau principle), with one electron per orbital before pairing (Hund's rule). A photoelectron spectrum (PES) shows peaks at binding energies: the peak closest to zero binding energy is the valence (outermost) shell, and each peak's height equals the number of electrons in that subshell. This is how PES proves the shell model — each subshell gets its own peak.
Periodic trends
Effective nuclear charge (Zₑff) grows left-to-right, so ionization energy and electronegativity increase across a period and decrease down a group (electrons are farther from the nucleus). Atomic radius shrinks across a period and grows down a group. Successive ionization energies jump dramatically when core electrons are reached — the jump reveals the group number.
Quantitative skill-set
Grams → moles via molar mass; moles → atoms via Avogadro's number; % composition from formula mass; empirical formula from % composition (assume 100 g, divide by atomic mass, normalize to whole numbers).
Exam traps
Ionization energy removes electrons one at a time from a gaseous atom. Noble gases have full valence shells — high IE, near-zero EN. Chromium and copper half-fill or fill their d subshells (4s¹3d⁵, 4s¹3d¹⁰).