Unit 1 covers the structure of atoms, electron configurations, and periodic trends — the foundation for understanding chemical bonding and reactivity throughout the AP Chemistry course.
Atom: nucleus (protons + neutrons) + electron cloud. Electron configuration: filling order (Aufbau), 1s 2s 2p 3s 3p 4s 3d 4p. Hund\'s rule: fill orbitals singly before pairing. Pauli: max 2e⁻ per orbital, opposite spins. PES data: peaks correspond to subshells; relative heights show number of electrons. Ionisation energies: confirm electron configuration.
Atomic radius: decreases across period (more protons, same shell), increases down group (more shells). Ionisation energy: increases across (harder to remove), decreases down (further, more shielding). Electronegativity: increases across and up. Electron affinity: generally more negative across period. Metallic character: increases down and left. Anomalies: IE drops from Be→B (2s→2p) and N→O (pairing energy).
Photoelectron spectroscopy (PES) provides experimental evidence for electron configurations. It shows: (1) the number of subshells (each peak), (2) the number of electrons in each subshell (peak heights), (3) the binding energy of each subshell (peak position — higher energy = closer to nucleus). This is physical proof that energy levels are quantised. Electron configuration is a theoretical model; PES data confirms it experimentally. On the AP exam, you need to interpret PES spectra to identify elements and connect peaks to subshells.
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