GENERAL CHEMISTRY · ATOMIC FOUNDATIONS

Count identity.
Bound the evidence.

Learn Sub-atomic Particles and Atomic Theory with a particle inventory and an observation-to-claim ledger, then retrieve the ideas with eight randomized five-choice questions.

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2guided lessons

8practice questions

6study objectives

5choices per item

A repeatable atomic routine

Identify. Inventory. Infer. Audit.

01Identify
Name the element from proton count before using mass or charge.
02Inventory
Separate protons, neutrons, and electrons using A, Z, and charge.
03Infer
Connect an observation only to the atomic-model claim it supports.
04Audit
Check charge, whole-number particle counts, abundance sum, and average range.

Two prerequisite-first lessons

Evidence before notation.

Particle identity supports every later electron, orbital, bond, and geometry model. Keep measured observation separate from the conclusion it can justify.

LESSON 1 · 14 MIN

Study + retrieve

Subatomic particles with an identity ledger

Compare subatomic particles, calculate a nuclide’s particle inventory, and distinguish isotope, ion, and element changes.

ESSENTIAL QUESTIONWhich particle count controls element identity, isotope identity, and charge?

³⁷Cl⁻ identity ledger

Particle counts from Z = 17, A = 37, and charge = −1
Quantity	Rule	Calculation	Count
Protons	Z	17	17
Neutrons	A − Z	37 − 17	20
Electrons	Z − charge	17 − (−1)	18

Identity auditElement: Cl · isotope: 37 · charge: −1

STUDY TABLE · CAPTION INCLUDED

Separate location, charge, and mass

Protons and neutrons occupy the nucleus and each contribute about one atomic mass unit; electrons occupy the surrounding quantum states and carry −1 charge with much smaller mass.

Proton +1 · neutron 0 · electron −1
Nucleons carry nearly all atomic mass

Build the inventory

Atomic number gives protons, mass number minus atomic number gives neutrons, and ionic charge determines the electron difference.

n = A − Z
e = Z − charge

Name the changed identity

Change protons and the element changes; change neutrons and the isotope changes; change electrons and the ion charge changes.

Element follows protons
Ion follows electron imbalance

Worked example

For chlorine-37 with a −1 charge and atomic number 17, find protons, neutrons, and electrons.

1
Protons equal atomic number: 17.
2
Neutrons equal 37 − 17 = 20.
3
A −1 charge means one more electron than protons: 18 electrons.

Conclusion³⁷Cl⁻ contains 17 protons, 20 neutrons, and 18 electrons.

Close the notes first

Retrieve the model.

01Which particle count fixes the element?

The proton count.

Atomic number is defined by the number of protons.

02How does a +2 charge change electron count?

The ion has two fewer electrons than protons.

Positive charge reflects electron loss.

03Two atoms share Z but differ in A. What are they?

Isotopes of the same element.

They share protons but differ in neutrons.

LESSON 2 · 15 MIN

Study + retrieve

Atomic models from evidence and abundance

Connect landmark observations to atomic-model revisions, interpret nuclide quantities, and calculate an abundance-weighted atomic mass.

ESSENTIAL QUESTIONWhat claim is actually supported by each experiment or isotope table?

Observation-to-claim ledger

Atomic-model claims supported by landmark observations
Observation	Supported claim	Does not establish
Cathode rays deflect as negative matter	Atoms contain negative subatomic particles	Nuclear size
Most alpha particles pass through foil	Most atomic volume is open	Every particle path
A few alpha particles deflect strongly	Positive charge and mass are concentrated	Electron orbitals

Two-isotope weighted-average check
Isotope mass	Fraction	Contribution
35 u	0.750	26.25 u
37 u	0.250	9.25 u
Weighted average		35.50 u

Range audit35.50 u lies between 35 u and 37 u

STUDY TABLE · CAPTION INCLUDED

Match evidence to the model

Cathode rays supported negative particles within atoms; rare large alpha-particle deflections supported a small, dense, positive nucleus and mostly empty atomic volume.

Observation before conclusion
Rare deflection matters

Keep A, Z, and average mass distinct

Z counts protons, A counts protons plus neutrons for one nuclide, and the periodic-table decimal is typically an isotope-abundance-weighted average.

A is a whole-number nuclide label
Average mass may be decimal

Weight before adding

Convert each abundance to a fraction, multiply it by that isotope’s mass, then sum all contributions and check that the result lies within the isotope range.

Σ fraction = 1
Average leans toward the abundant isotope

Worked example

An element is 75.0% isotope-35 and 25.0% isotope-37. Using the stated mass numbers, find its average atomic mass.

1
Convert percentages to 0.750 and 0.250.
2
Calculate contributions: 35(0.750) = 26.25 and 37(0.250) = 9.25.
3
Add the contributions: 26.25 + 9.25 = 35.50.

ConclusionThe stated two-isotope model gives an average atomic mass of 35.5 u to three significant figures.

Close the notes first

Retrieve the model.

01What did rare large alpha-particle deflections imply?

Positive charge and most mass are concentrated in a small nucleus.

Diffuse positive charge would not produce those large-angle deflections.

02Why is a periodic-table mass often not a whole number?

It is a weighted average over isotope masses and abundances.

It does not describe the mass number of one typical atom.

03Where must a valid two-isotope weighted average lie?

Between the two isotope masses.

A convex weighted average cannot fall outside the contributing range.

All eight Atomic Foundations problems

Choose the governing ledger.

Question order and all five answer options shuffle each time. Reports automatically identify the exact question if you find a problem.

8 PRACTICE QUESTIONS

Retrieve before you review.

Question order and all five answer options are shuffled when you begin. The correct answer stays attached to the same underlying choice.

Continue the sequence

Two foundations, then three Electronic Structure topics.

This route covers Sub-atomic Particles and Atomic Theory. Continue to Electronic Structure for Quantum Theory, Orbital Types, and Electron Configuration.

Use raw accuracy to choose what to review next—not as an official DAT score prediction.