AP Chemistry · Handsworth Secondary 2026–27
Unit 1 · Study Guide
Dr. Ras Mulinta
Atomic Structure & Properties
Exam-focused review
A one-page map of what the Unit 1 test (and the AP exam) expects. This is a checklist, not a re-teach, if a line doesn't click, go back to that section of the notes package. A periodic table is provided.
Must be able to do
- 1.1 n = m/M and N_A = 6.022×10²³convert between mass, moles, and particles in either direction.
- 1.2 Average atomic mass = Σ(isotope mass × fractional abundance). Work it forwards (find M̄) and backwards (find abundances).
- 1.3 Percent composition → empirical formula → (with molar mass) molecular formula.
- 1.4 Use elemental composition to find a component's amount or a sample's purity.
- 1.5 Write ground-state configurations of atoms and ions (remove outer-s electrons first for cations); use F ∝ q₁q₂/r² to compare how tightly electrons are held.
- 1.6 Translate a PES spectrum ↔ electron configuration; peak position = binding energy, peak height ∝ electron count.
- 1.7 Explain radius, ionization energy, electron affinity, and electronegativity trends with Z_eff, shielding, and distance.
- 1.8 Predict ion charges and likely formulas from valence electrons / group number.
The big idea that ties it together
Coulomb's law F ∝ q₁q₂/r² is the engine of the whole unit. Ionization energy, PES peak positions, and every periodic trend are the same story: how strongly does this nucleus hold this electron?set by nuclear charge, distance, and shielding.
Don't waste time on (excluded by the CED)
Quantum numbers (n, ℓ, mℓ, ms) · mass spectra of multiple elements or non-singly-charged ions · the aufbau exceptions (Cr, Cu). Know the patterns, skip the edge cases.
Traps that cost marks
Mass number vs average atomic mass: mass number (protons + neutrons) is a whole number for one isotope; the periodic-table mass is a weighted average of all isotopes, don't report one for the other.
PES height vs position: peak height ∝ number of electrons in that subshell; peak position (binding energy) shows how tightly they're held, high binding energy = closest to the nucleus (1s), not "most electrons."
Weighted average, not simple average: average atomic mass = Σ(mass × fractional abundance). Averaging the isotope masses straight (ignoring abundance) is the classic isotope-mass mistake.
Shielding vs effective nuclear charge: shielding is inner electrons blocking the pull; Zeff = nuclear charge − shielding. Explain trends with the net Zeff, not shielding alone.
Coulomb's law direction: closer distance and higher charge = stronger attraction = higher binding energy / ionization energy. Don't flip it, bigger, farther electrons are held more loosely.
Cations lose outer-s first: for transition-metal ions remove the 4s electrons before the 3d (e.g. Fe³⁺ = [Ar]3d⁵, not [Ar]4s²3d³), a frequent electron-configuration slip.
Quick self-check (answers in your head, then verify)
- Moles in 11.0 g of CO₂?
- Average atomic mass of an element that is 60.0% of mass 69.0 and 40.0% of mass 71.0?
- Empirical formula of a compound 75.0% C, 25.0% H by mass?
- Ground-state configuration of Mn²⁺ (Mn is Z = 25)?
- Which has the higher first ionization energy: P or S? (Watch the half-filled subshell.)
- A PES spectrum shows heights 2, 2, 6, 2, 5 (high → low energy). What element?
Check yourself: 1) 0.250 mol 2) 69.8 3) CH₄ 4) [Ar] 3d⁵ 5) P (its half-filled 3p³ is extra stable, so S is slightly easier to ionize) 6) configuration 1s²2s²2p⁶3s²3p⁵ = 17 e⁻ → chlorine.