AP Chemistry Unit 1: Atomic Structure and Properties Notes

Introduction

Chemists count particles by measuring mass. The mole connects a laboratory-scale sample to atoms, ions, molecules, or formula units, while composition data reveal the ratios inside pure substances and mixtures.

Electron configurations organize electron occupancy. Mass spectra, photoelectron spectra, and periodic measurements then supply evidence for isotope abundance, subshell population, attraction to the nucleus, and common ion formation.

Every model in this unit has a boundary: an average atomic mass is not one atom, an orbital is not a path, a trend is not an exception-free law, and an ionic formula is a charge-balanced ratio rather than a molecule.

Amount

mass → moles → specified entities

Composition

element or component measurements → ratios

Electronic structure

electron count → configuration → spectroscopic evidence

Periodic behavior

nuclear charge, shielding, and shells → trends and ion formation

Unit map

Identity across representations

Use this sequence to choose the next representation or calculation in Unit 1.

measured amountcompositionelectron structureperiodic evidence

evidence → model → calculation → claim

Read this figure without the graphic

A four-stage unit map: measured amount to composition to electron structure to periodic evidence.

Stage	Purpose
1	measured amount
2	composition
3	electron structure
4	periodic evidence

Reference

Formula and theorem sheet

Match each expression with the condition that makes it valid.

Current exam guidance: Aligned to the current 2026–27 AP Chemistry course framework and exam scope.

1.1

Moles and Molar Mass

How can a measured mass tell us how many microscopic entities are present?

Short answer

Convert mass to moles with the substance’s molar mass, then convert moles to a count of explicitly named entities with the Avogadro constant.

Know firstChemical formulas · Dimensional analysis · Scientific notation

Mass becomes a count through amount

A balance measures grams, but chemical change occurs among particles. Molar mass is the bridge: its units, $\mathrm{g\,mol^{-1}}$ , make $m/M$ an amount in moles. One mole contains exactly $6.02214076\times10^{23}$ specified elementary entities. The word specified matters. A mole of atoms, molecules, ions, formula units, and electrons contains the same number of entities, but not the same kind of entity.

Read the formula before calculating. One mole of a molecular substance contains one mole of molecules, while its subscripts determine how many moles of each atom are present. An ionic solid is counted in formula units, not molecules. In a mixture, a total mass is not enough to count one component until its composition or component mass is known.

Build the conversion so units cancel visibly: grams multiply by moles per gram, then by entities per mole. Exact subscripts and the exact SI value of $N_A$ do not limit significant figures. The measured mass and the molar masses used in the calculation control the final precision.

Chemistry model

From measured mass to particles

Track units through mass, moles, and specified entities.

01Measured sample18.015 g H₂O

Start with the substance and its measured mass.

02Chemical amount1.000 mol H₂O

Divide by 18.015 g mol⁻¹.

03Specified entities6.022 × 10²³ molecules

Multiply moles by Avogadro’s constant.

m → n → N

Read this figure without the graphic

From measured mass to particles. The figure connects 18.0 g H₂O, 0.999 mol, 6.02 × 10²³ molecules so the relationship can be read without relying on color alone.

Representation	What to read
18.0 g H₂O	starting evidence or quantity
0.999 mol	intermediate relationship
6.02 × 10²³ molecules	result or conclusion

Formula

The amount chain

n=\frac{m}{M},\qquad N=nN_A

Mass divided by molar mass gives amount; amount multiplied by the Avogadro constant gives a count.

Use when: The sample composition and counted entity must be defined, with mass and molar-mass units compatible.
Do not assume: A bulk mixture mass cannot identify one component’s particle count without composition information.
Variables: $m$ : sample mass (g) · $M$ : molar mass (g mol^-1) · $n$ : amount (mol)
Common misuse: Treating molar mass, mole count, and particle count as interchangeable numbers.

Keep the entity attached to the number

Stage	Quantity and unit	Question answered
Mass	$m$ in g	How much does the sample weigh?
Amount	$n=m/M$ in mol	What amount of substance is present?
Count	$N=nN_A$	How many named entities are present?
Formula factor	subscript × $N$	How many atoms or ions of one kind?

Set up a defensible conversion

Name the substance and the entity being counted.
Calculate molar mass from the full formula.
Convert mass to moles and show unit cancellation.
Apply any formula subscript, then multiply by $N_A$ .
Round from measured inputs and label the entity.

Keep these three ideas

Name the entity before calculating.
Use $m/M$ for moles and $nN_A$ for entity count.
Let measured quantities—not exact counting constants—set precision.

1.2

Mass Spectra of Elements

Why is the atomic mass on the periodic table usually not a whole number?

Short answer

It is a population-weighted mean of the naturally occurring isotope masses, using each isotope’s fractional abundance.

Know firstIsotope notation · Weighted means · Percent-to-decimal conversion

A spectrum separates isotope signals

A mass spectrum plots detector signal against mass-to-charge ratio, $m/z$ . In the AP single-element model, the sample produces singly charged monatomic ions, so each peak position identifies an isotope mass and each normalized peak area or intensity represents its relative abundance. Read the axis and the stated ion model before assigning peaks.

Average atomic mass is not the simple mean of the peak positions unless the peaks have equal abundance. Convert percentages to fractions, multiply each isotope mass by its fraction, and add the contributions. The result must lie between the smallest and largest isotope masses. It describes the sampled population; it need not equal the mass of any one atom.

The model has a deliberate boundary. Molecular fragments, multiple elements, or multiply charged ions can produce other peak patterns, but those complications are outside this topic’s spectrum interpretation. Peak height also does not tell you neutron count; the horizontal coordinate carries mass information.

Chemistry model

Isotope peaks determine average mass

Change the heavier-isotope abundance and watch both peak heights and the weighted mean move.

Heavy-isotope abundance24 %

³⁵Cl fraction: 0.76
³⁷Cl fraction: 0.24
Weighted mass: 35.448 u

m̄ = Σfᵢmᵢ

Read this figure without the graphic

Isotope peaks determine average mass. The figure connects lighter isotope, heavier isotope, weighted mean so the relationship can be read without relying on color alone.

Representation	What to read
lighter isotope	starting evidence or quantity
heavier isotope	intermediate relationship
weighted mean	result or conclusion

Large time-of-flight mass spectrometer with an enclosed ion source, flight tube, and detector housing. — A mass spectrometer is a measurement chain
The instrument ionizes a sample, separates ions by mass-to-charge behavior, and records a detector signal. The peak plot in the lesson is the output of that chain—not a photograph of atoms.
Use this visual to see
Connect sample introduction, ion separation, and detection before interpreting isotope peaks.
What it cannot show
The housing hides the ion path and does not show calibration, charge state, or how the detector signal becomes a normalized spectrum.

Formula

Weighted average mass

\bar m=\sum_i f_i m_i,\qquad \sum_i f_i=1

Each isotope mass contributes in proportion to its fractional abundance.

Use when: Fractions are nonnegative, normalized to one, and refer to one element’s singly charged monatomic ions.
Do not assume: A plain average of peak positions ignores abundance.
Variables: $f_i$ : fractional abundance of isotope i (dimensionless) · $m_i$ : isotope mass (u)
Common misuse: Using percentages as if they were decimals or averaging peak positions without weights.

Synthetic three-isotope spectrum

$m/z$	Fraction	Mass contribution
24.00	0.7000	16.80 u
25.00	0.2000	5.000 u
26.00	0.1000	2.600 u
Total	1.0000	24.40 u

Keep these three ideas

Peak position identifies mass under the singly charged model.
Peak abundance supplies the weight.
A population average need not be an actual isotope mass.

1.3

Elemental Composition of Pure Substances

How does elemental analysis reveal the simplest formula of a pure compound?

Short answer

Convert each element’s measured mass to moles, divide by the smallest amount, and scale all ratios together to the smallest credible whole numbers.

Know firstMoles and molar mass · Mass percent · Ratios

Formulas record amount ratios, not mass ratios

A pure compound has a reproducible elemental composition. Measurements usually report each element’s mass or mass percent, but formula subscripts compare numbers of atoms. Convert every mass to moles before looking for a ratio. When percentages are given, a 100 g basis is convenient because each percentage becomes the same numerical mass in grams.

Divide all mole amounts by the smallest. Ratios close to integers can be rounded only within the measurement precision. Values near one-half, one-third, or another simple fraction require multiplying every ratio by the same small integer. Never round one component independently or force a doubtful value into an integer.

Finish with a back-check: calculate the mass percentages predicted by the empirical formula and compare them with the measurements. The empirical formula gives the smallest whole-number ratio. It does not determine molecular formula without a molar mass or equivalent evidence.

Chemistry model

Composition stays fixed in a pure compound

Connect one particle formula to atom counts and each element’s mass contribution.

2 H atoms2.016 g mol⁻¹

11.19% of the molar mass

1 O atom15.999 g mol⁻¹

88.81% of the molar mass

One H₂O molecule18.015 g mol⁻¹

The atom ratio stays 2:1 in every pure sample.

\%\,\mathrm{element}=\frac{\mathrm{element\ mass}}{\mathrm{molar\ mass}}

Read this figure without the graphic

Composition stays fixed in a pure compound. The figure connects formula unit, atom ratio, mass contribution so the relationship can be read without relying on color alone.

Representation	What to read
formula unit	starting evidence or quantity
atom ratio	intermediate relationship
mass contribution	result or conclusion

Formula

Composition relations

w_E=\frac{m_E}{m_{\text{sample}}},\qquad \%E=100w_E

Mass fraction reports how much sample mass comes from one element; mole conversion is still required for subscripts.

Use when: The analyzed sample is treated as a pure substance and elemental masses account for the sample within uncertainty.
Do not assume: Using mass percentages directly as subscripts confuses mass ratio with atom ratio.
Variables: $w_E$ : mass fraction of element E (dimensionless)
Common misuse: Rounding normalized mole ratios too early.

A complete ratio ledger

Element	Mass	Molar mass	Moles	Normalized ratio
A	40.00 g	20.00 g mol^-1	2.000 mol	1.000
B	60.00 g	30.00 g mol^-1	2.000 mol	1.000
Result	100.00 g	—	—	AB

From analysis to formula

Choose the measured mass basis.
Convert every element mass to moles.
Divide all amounts by the smallest.
Scale all ratios together if a simple fraction remains.
Recompute the predicted mass percentages.

Keep these three ideas

Convert mass to moles before assigning subscripts.
Scale every ratio by the same factor.
Back-check the predicted composition.

1.4

Composition of Mixtures

When can a bulk measurement determine the amounts of components in a mixture?

Short answer

A total balance plus an independent component-sensitive measurement can determine composition when the assumed response model is valid and the solution is physically possible.

Know firstMass conservation · Mass fractions · Linear equations

Mixtures need more than one constraint

A mixture contains components whose proportions can vary. Total mass provides one equation, $m_{tot}=m_A+m_B+\cdots$ , but one equation cannot determine several unknown component masses. A second measurement must respond differently to the components—for example, a calibrated additive signal—before the composition can be solved.

Choose a basis and state the model. For two components, write a mass balance and a second relation such as $P=w_AP_A+(1-w_A)P_B$ only when the measured property is known to combine linearly under those conditions. Solve, then require every fraction to lie from zero to one and reconstruct both observations.

Mass remains additive in a closed transfer, but volume or an instrumental response need not be. A homogeneous sample is uniform at the observed scale; that does not make it a pure substance or give it a fixed formula.

Chemistry model

A mixture needs a mass balance and another constraint

Separate total sample mass into component contributions and verify that fractions sum to one.

NaCl · 16.0 gsand · 9.0 g

Total sample: 25.0 g
NaCl mass fraction: 16.0/25.0 = 0.640
Sand mass fraction: 9.0/25.0 = 0.360

Mass balance alone gives one equation. A composition measurement supplies the second constraint needed to separate unknown component amounts.

Σwᵢ = 1

Read this figure without the graphic

A mixture needs a mass balance and another constraint. The figure connects component A, component B, total sample so the relationship can be read without relying on color alone.

Representation	What to read
component A	starting evidence or quantity
component B	intermediate relationship
total sample	result or conclusion

Formula

Mass-fraction constraints

m_{\text{tot}}=\sum_i m_i,\qquad w_i=\frac{m_i}{m_{\text{tot}}},\qquad \sum_i w_i=1

Component masses reconstruct the total and their fractions exhaust the composition.

Use when: The sample basis and components are defined and transfers are closed with respect to mass.
Do not assume: One total mass alone cannot identify several component masses.
Variables: $w_i$ : mass fraction of component i (dimensionless)
Common misuse: Assuming every measured property is additive like mass.

A 50.0 g two-component basis

Quantity	Component A	Component B	Check
Mass fraction	0.350	0.650	sum = 1.000
Mass	17.5 g	32.5 g	sum = 50.0 g
Physical requirement	$0\le w_A\le1$	$0\le w_B\le1$	both pass

Solve and test the model

Name the components and choose a total basis.
Write the mass balance.
Write one independent relation for each remaining unknown.
Solve and reject negative or greater-than-one fractions.
Reconstruct the measurements and state the additivity assumption.

Keep these three ideas

A total balance is one constraint, not a full composition.
Use an independent response only when its model is justified.
Reject solutions outside physical fraction bounds.

1.5

Atomic Structure and Electron Configuration

What information does an electron configuration encode about an atom or ion?

Short answer

It records the ground-state occupancy of allowed subshells for a species with a specified electron count; it does not draw electron paths.

Know firstAtomic number and mass number · Ionic charge · Electrostatic attraction

Count first, arrange second

Atomic number $Z$ fixes the element because it counts protons. Mass number counts protons plus neutrons, while charge compares proton and electron counts. In elementary-charge units, $N_e=Z-z$ : a positive ion has fewer electrons than protons and a negative ion has more.

Electron configurations summarize how those electrons occupy subshells in a ground-state model. An s subshell holds 2 electrons, p holds 6, d holds 10, and f holds 14. Within an orbital, at most two electrons occupy opposite-spin states; within a set of equal-energy orbitals, the basic model places one electron in each before pairing. The usual filling order is a guide with documented ground-state exceptions.

Configurations support predictions about valence electrons, spectra, and ion formation, but the notation is not a map of circular paths. When forming transition-metal ions, do not simply erase the last symbols written; remove electrons according to the species’ outer energetic subshell convention.

Chemistry model

Electron configurations record occupancy

Read the energy ladder, orbital boxes, and configuration as three views of the same electron count.

↑↑↑

↑↓

Electron count: 7 electrons
Configuration: 1s² 2s² 2p³
Orbital rule: Fill equal-energy 2p orbitals singly before pairing.

Nₑ = Z − z

Read this figure without the graphic

Electron configurations record occupancy. The figure connects energy ladder, orbital occupancy, configuration so the relationship can be read without relying on color alone.

Representation	What to read
energy ladder	starting evidence or quantity
orbital occupancy	intermediate relationship
configuration	result or conclusion

Formula

Electron-count ledger

N_e=Z-z,\qquad N_n=A-Z

Electron count follows nuclear charge and ionic charge; neutron count follows mass and atomic numbers.

Use when: The ionic charge $z$ is expressed in elementary-charge units with its sign.
Do not assume: Treating a +2 charge as two additional electrons reverses the physical meaning.
Variables: $Z$ : atomic number and proton count (count) · $A$ : mass number (count) · $z$ : signed ionic charge number (elementary charge units)
Common misuse: Deleting the last-written subshell without considering ionization ordering.

Subshell capacities

Subshell	Orbitals	Maximum electrons	Model cue
s	1	2	one orbital
p	3	6	three degenerate orbitals
d	5	10	five orbitals
f	7	14	seven orbitals

Build a configuration without losing the species

Use atomic number and charge to determine total electrons.
Fill subshells in the appropriate ground-state order.
Respect orbital capacity and singly occupy degenerate orbitals before pairing.
Sum every superscript to check the electron count.
For ions, apply the correct removal convention and recheck charge.

Keep these three ideas

Element identity comes from proton count.
Charge determines the electron-count adjustment.
Configuration notation encodes occupancy, not a literal trajectory.

1.6

Photoelectron Spectroscopy

How can a photoelectron spectrum support a claim about electron configuration?

Short answer

Peak groups locate occupied subshells by binding energy and represent their populations by relative area, while photon energy conservation links binding and electron kinetic energy.

Know firstElectron configurations · Energy conservation · Graph axes

Binding energy and population are different readings

In photoelectron spectroscopy, incident radiation ejects electrons. In the simplified energy accounting, the photon’s energy is divided between the binding energy needed to remove the electron and the kinetic energy measured after ejection. More tightly bound electrons require larger binding energy.

A spectrum groups signals by subshell. Peak position communicates binding energy; relative peak area or intensity under matched conditions communicates how many electrons occupy that subshell. Read the axis direction because binding-energy axes are sometimes displayed in reverse. The tallest peak is not automatically the highest-binding-energy peak.

Use a configuration as a consistency check: assigned subshell populations should sum to the species’ electron count. A PES peak is evidence about electron energy, not a picture of an orbital. Calibration, charging, and acquisition conditions can shift measured positions, so compare spectra only under a stated basis.

Chemistry model

PES position and area answer different questions

Use peak position for binding energy and peak area for subshell population.

higher binding energy ←→ lower binding energy

1s²2s²2p⁶3s²

Peak position: How tightly the electrons are bound
Relative area: How many electrons occupy that subshell

Ephoton = BE + KE

Read this figure without the graphic

PES position and area answer different questions. The figure connects core peak, valence peak, relative population so the relationship can be read without relying on color alone.

Representation	What to read
core peak	starting evidence or quantity
valence peak	intermediate relationship
relative population	result or conclusion

A zinc sample producing a pale blue-green flame in a laboratory flame test. — Visible color is one kind of spectroscopic evidence
Excited zinc atoms emit discrete wavelengths as electrons fall to lower energy states. Photoelectron spectroscopy asks a different question: how much energy is required to remove an electron.
Use this visual to see
Separate the observed light of an emission experiment from the binding-energy evidence in a PES spectrum.
What it cannot show
Camera exposure changes perceived flame color, and a flame photograph cannot identify subshell populations or measure electron binding energies.

Formula

Photoelectron energy balance

E_{\text{photon}}=BE+KE_{e^-}

Photon energy supplies the binding energy and the measured kinetic energy of the ejected electron.

Use when: All terms use a consistent per-electron energy unit and the event has enough photon energy to eject the electron.
Do not assume: A binding energy greater than the incident photon energy is impossible in this simplified event.
Variables: $BE$ : electron binding energy (eV per electron) · $KE$ : ejected-electron kinetic energy (eV per electron)
Common misuse: Reading peak height as binding energy.

Synthetic PES reading

Peak group	Relative binding	Relative area	Occupancy inference
A	highest	2	2 electrons
B	middle	2	2 electrons
C	lowest	6	6 electrons
Total	—	10	10 electrons

Read a PES plot in the right order

Read the binding-energy axis direction and units.
Count peak groups before assigning subshells.
Use relative areas for electron populations.
Use positions for relative binding strength.
Check the inferred populations against one valid configuration.

Keep these three ideas

Position represents binding energy; area represents population.
Read the axis before ranking peaks.
A PES signal is evidence about energy, not orbital shape.

1.7

Periodic Trends

What physical reasoning explains periodic changes in radius and ionization energy?

Short answer

Compare occupied shells, proton count, shielding, and electron configuration; the balance of nuclear attraction and electron distance produces trends plus specific exceptions.

Know firstElectron configuration · Coulombic attraction · PES binding energy

Use a mechanism, not an arrow alone

Across a period, electrons are added mainly within the same principal shell while proton count rises. Shielding does not rise enough to cancel the greater nuclear attraction, so atomic radius generally decreases and first ionization energy generally increases. Down a group, a new occupied shell places valence electrons farther from the nucleus and increases shielding, so radius generally increases.

These are patterns, not exception-free formulas. Subshell energy and electron pairing can interrupt a simple ionization-energy rise. State the relevant configuration when a measured value breaks the broad trend. Radius also depends on how it is defined and measured; an atom is not a hard sphere with one universal edge.

For an isoelectronic series, electron count and shell arrangement are held constant. The species with more protons attracts the same number of electrons more strongly and is generally smaller. This comparison shows why “more electrons means bigger” is not a complete rule.

Chemistry model

Periodic trends need a physical mechanism

Compare shell count, nuclear charge, and radius across a period and within an isoelectronic series.

Period 3NaMgAlSiPSCleffective nuclear attraction increases →

10-electron seriesO²⁻largestF⁻Na⁺Mg²⁺smallest

Across a period, added protons increase attraction while electrons enter the same principal shell. In an isoelectronic series, the species with more protons is smaller.

attraction ∝ charge / distance²

Read this figure without the graphic

Periodic trends need a physical mechanism. The figure connects nuclear charge, shielding, radius so the relationship can be read without relying on color alone.

Representation	What to read
nuclear charge	starting evidence or quantity
shielding	intermediate relationship
radius	result or conclusion

Choose the controlling comparison

Comparison	Main evidence	Typical inference	Required caution
Across a period	higher $Z$ in similar shell	smaller radius; higher IE	configuration exceptions
Down a group	added occupied shell	larger radius; lower IE	measured-property variation
Isoelectronic ions	same electrons, different $Z$	higher $Z$ → smaller	charges must be stated
Successive IE	large energy jump	core removal begins	identify number removed

Justify a ranking

Identify whether the species share an occupied shell or electron count.
Compare proton count and shielding.
State the predicted change in attraction and electron distance.
Check for a subshell or paired-electron exception.
Connect the mechanism to the requested measured property.

Keep these three ideas

Across-period attraction usually strengthens within a similar shell.
Down-group shells and shielding usually increase radius.
Configuration and charge explain exceptions and ionic comparisons.

1.8

Valence Electrons and Ionic Compounds

How do valence electrons and charge conservation constrain an ionic formula?

Short answer

Common ion charges follow electron-configuration patterns, and the smallest whole-number ion ratio must make the formula unit electrically neutral.

Know firstValence configurations · Ionic charge · Whole-number ratios

An ionic formula is a neutral ratio

Main-group valence configurations help predict common monatomic ion charges. Losing electrons produces a cation; gaining electrons produces an anion. The octet pattern is a useful description for many main-group ions, but it is not a universal motive and it does not determine every transition-metal charge.

An ionic formula reports the smallest whole-number ratio whose signed charges sum to zero. For ions with charges $+2$ and $-3$ , three cations and two anions give $3(+2)+2(-3)=0$ , so the ratio is 3:2. Subscripts count ions; they are not the charge labels themselves.

The formula unit describes composition in an extended ionic lattice. It should not be pictured as one isolated ionic molecule. Conductivity also requires mobile charged particles: ions fixed in a solid lattice do not move through the sample as they can in a molten phase or suitable solution.

Chemistry model

Ionic formulas are neutral ratios

Balance cation and anion charges, then connect the formula unit to an extended lattice.

2 Al³⁺+6+3 O²⁻−6=Al₂O₃ · net 0

The formula is the smallest whole-number charge-balanced ratio. It does not describe an isolated molecule inside the ionic solid.

Σνᵢzᵢ = 0

Read this figure without the graphic

Ionic formulas are neutral ratios. The figure connects 3 cations × +2, 2 anions × −3, net charge 0 so the relationship can be read without relying on color alone.

Representation	What to read
3 cations × +2	starting evidence or quantity
2 anions × −3	intermediate relationship
net charge 0	result or conclusion

Formula

Charge neutrality

\sum_i \nu_i z_i=0

Each ion’s signed charge multiplied by its formula-unit count must sum to zero.

Use when: Ion charges are known and subscripts are reduced to the smallest whole-number ratio.
Do not assume: Crossing charge magnitudes without reducing can leave a nonminimal formula.
Variables: $\nu_i$ : whole-number count of ion i (count) · $z_i$ : signed ionic charge (elementary charge units)
Common misuse: Writing charge numbers as subscripts without checking the signed sum.

Charge tiles for a +2/−3 pair

Ion type	Count	Charge each	Charge contribution
Cation	3	+2	+6
Anion	2	−3	−6
Formula unit	3:2	—	0

Write and verify the formula

Identify the ion symbols and signed charges.
Find the smallest whole-number counts that cancel charge.
Write the counts as subscripts and omit subscript 1.
Reduce any common factor.
Recalculate the total signed charge from the finished formula.

Keep these three ideas

Charges belong to ions; subscripts count their ratio.
The signed total must be zero for a neutral formula unit.
Ionic solids are extended lattices, not isolated molecules.

Cumulative review

What should be ready after this unit

This is a study map, not a quiz. Explain each item aloud, then return to the linked topic when the reasoning is incomplete.

Recognize immediately

Specified entity
Weighted isotope peak
Subshell peak group
Isoelectronic comparison
Charge-balanced ratio

Reconstruct

Mass-to-particle unit chain
Empirical-formula ratio
Electron-count ledger
Periodic-trend explanation

State or verify

Defined sample composition
Fractions sum to one
PES axis direction read
Species and charge stated

Catch in your own work

Did the units cancel?
Did I count the correct entity?
Did I use evidence rather than a trend slogan?
Is total charge zero?

Introduction

Identity across representations

Formula and theorem sheet

Mass becomes a count through amount

From measured mass to particles

The amount chain

Keep the entity attached to the number

Set up a defensible conversion

Keep these three ideas

A spectrum separates isotope signals

Isotope peaks determine average mass

A mass spectrometer is a measurement chain

Weighted average mass

Synthetic three-isotope spectrum

Keep these three ideas

Formulas record amount ratios, not mass ratios

Composition stays fixed in a pure compound

Composition relations

A complete ratio ledger

From analysis to formula

Keep these three ideas

Mixtures need more than one constraint

A mixture needs a mass balance and another constraint

Mass-fraction constraints

A 50.0 g two-component basis

Solve and test the model

Keep these three ideas

Count first, arrange second

Electron configurations record occupancy

Electron-count ledger

Subshell capacities

Build a configuration without losing the species

Keep these three ideas

Binding energy and population are different readings

PES position and area answer different questions

Visible color is one kind of spectroscopic evidence

Photoelectron energy balance

Synthetic PES reading

Read a PES plot in the right order

Keep these three ideas

Use a mechanism, not an arrow alone

Periodic trends need a physical mechanism

Choose the controlling comparison

Justify a ranking

Keep these three ideas

An ionic formula is a neutral ratio

Ionic formulas are neutral ratios

Charge neutrality

Charge tiles for a +2/−3 pair

Write and verify the formula

Keep these three ideas

What should be ready after this unit

Recognize immediately

Reconstruct

State or verify

Catch in your own work

Practice AP Chemistry in the real test engine.