Atomic Structure & Properties

What this unit covers

The topics below follow the published Chemistry course framework for Unit 1. This unit is worth 7–9% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• The Mole & Molar Mass12 min · 3 objectivesExplain what a mole counts and why chemists use it · Convert between grams, moles, and number of particles · Calculate the molar mass of any compound from the periodic table
• Isotopes & Mass Spectrometry11 min · 3 objectivesDistinguish atomic number, mass number, and isotopes · Compute average atomic mass from isotopic abundances · Read a mass spectrum to identify isotopes and their abundances
• Electron Configuration14 min · 3 objectivesOrder subshells by energy using the Aufbau principle · Write full and noble-gas electron configurations · Apply the Pauli exclusion principle and Hund’s rule
• Periodic Trends13 min · 3 objectivesExplain trends in atomic radius, ionization energy, and electronegativity · Use effective nuclear charge (Zₑff) to justify each trend · Rank atoms and ions by size and reactivity
• Photoelectron Spectroscopy (PES)14 min · 3 objectivesExplain how PES uses photon energy and electron kinetic energy to measure binding energies · Read a PES spectrum: peak position as subshell binding energy, peak height as electron count · Match a PES spectrum to an element and its electron configuration
• Coulomb's Law & Periodic Trends (Quantitative)14 min · 3 objectivesState Coulomb’s law and connect Coulombic attraction to effective nuclear charge (Zₑff) · Use charge and distance to rank ionization energies and atomic radii quantitatively · Interpret successive ionization energies, reading large jumps as shell boundaries

Formulas in Unit 1

Every term in Unit 1

All 21 terms we publish for Atomic Structure & Properties, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Empirical vs molecular formula

Empirical is the smallest whole-number ratio; molecular is the actual count. Molecular mass divided by empirical mass gives the multiplier.

Mole and Avogadro's number

One mole is 6.022 × 10²³ particles. It converts between the atomic scale we reason about and the gram scale we can weigh.

Molar mass

Grams per mole, numerically equal to the atomic or formula mass in amu. The bridge between mass measured and moles reacted.

Percent composition

Mass of an element divided by the total formula mass, times 100. The starting point for finding an empirical formula from combustion data.

Mass spectrometry

Separates isotopes by mass-to-charge ratio. Peak positions give isotopic masses and peak heights give relative abundances.

Average atomic mass

Sum of each isotope's mass times its fractional abundance — a weighted average, which is why it is rarely a whole number.

Coulomb's law in atoms

Force ∝ q₁q₂/r². Greater nuclear charge or shorter distance means stronger attraction, which explains ionization energy and atomic radius trends.

Photoelectron spectroscopy

Peak binding energy indicates which subshell an electron came from; peak height gives how many electrons are in it. Reads electron configuration directly.

Electron configuration

Order by increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p. Cr and Cu are exceptions, taking a half-filled or filled d subshell.

Effective nuclear charge (Zeff)

The net positive charge an outer electron feels after inner-shell shielding. Rises across a period, which drives every periodic trend.

Shielding

Inner electrons repel outer ones and reduce the nuclear pull they feel. Core electrons shield strongly; electrons in the same shell shield poorly.

Atomic radius trend

Decreases across a period as Zeff rises pulling electrons in; increases down a group as a new shell is added.

Ionic radius

Cations are smaller than their parent atoms (lost a shell, less repulsion); anions are larger (added electrons increase repulsion at constant nuclear charge).

Ionization energy trend

Increases across a period and decreases down a group. A large jump between successive ionizations marks the start of a new inner shell.

Electronegativity

Tendency to attract shared electrons in a bond. Increases up and to the right; fluorine is highest.

Valence electrons

Outermost-shell electrons, equal to the main-group number. They determine bonding behavior almost entirely.

Isotopes

Same element, different neutron count. Chemical behavior is essentially identical because it depends on electrons, not the nucleus.

Reading a PES spectrum

Peaks left to right run from highest to lowest binding energy, matching 1s, 2s, 2p and so on. A 2p peak is three times the height of a 2s peak when both are full.

Why the first ionization energy of oxygen is below nitrogen

Oxygen's fourth 2p electron must pair in an occupied orbital, and that added electron-electron repulsion makes it easier to remove.

Photon energy

E = hν = hc/λ. Shorter wavelength means higher energy, which is why ultraviolet light breaks bonds that visible light cannot.

Absorption and emission spectra

Discrete lines occur because electron energy levels are quantized, so only photons matching an energy gap are absorbed or emitted.

What examiners penalize here

• On the AP exam, moles are the hub of almost every quantitative problem. Train the reflex: grams → (÷ molar mass) → moles → (× ratio) → moles of target → (× molar mass or Avogadro) → answer.
• AP loves connecting configuration to the periodic table: the s-block is groups 1–2, the d-block is the transition metals, and the p-block is groups 13–18. An element’s valence configuration is readable straight off its position.
• Whenever a free-response asks you to *justify* a trend, name the mechanism — “higher Zₑff” or “additional shell / more shielding.” Stating the trend alone rarely earns the point; the reasoning does.
• AP frequently shows two spectra and asks which belongs to the larger atom or which subshell was affected by ionization. Anchor every answer in the two rules: peak position = binding energy (tighter hold, higher Zₑff, shifts left), peak area = electron count. Removing an electron shrinks the corresponding peak’s height.
• When a free-response asks you to *justify* a ranking, cite both Coulomb levers explicitly: state whether Zₑff or r changed and in which direction, then conclude about the force. “Higher Zₑff, same shell → stronger attraction → higher IE / smaller radius” is the sentence that earns the point.

Practice Chemistry

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 9 units of AP Chemistry

1. Unit 1 · Atomic Structure & Properties
2. Unit 2 · Molecular and Ionic Compound Structure and Properties
3. Unit 3 · Intermolecular Forces and Properties
4. Unit 4 · Chemical Reactions
5. Unit 5 · Kinetics
6. Unit 6 · Thermodynamics
7. Unit 7 · Equilibrium
8. Unit 8 · Acids & Bases
9. Unit 9 · Applications of Thermodynamics