Count protons, neutrons and electrons separately
You will be able to: Calculate subatomic counts from atomic number, mass number and charge.
What changes when an atom becomes an isotope or an ion?
A sodium atom has 11 protons. Removing one electron makes Na⁺, but it is still sodium. Adding a neutron instead changes its isotope while leaving the element unchanged.
A useful starting point: Elemental analysis can test a stated purity model →
Words and symbols before equations
- Atomic number Z
- Proton count, which identifies the element.
- Mass number A
- Protons plus neutrons in a particular isotope.
- Charge q
- Net charge in elementary-charge units: proton count minus electron count.
- Ion
- An atom or group with unequal positive and negative charge.
What this picture assumes
Sodium nucleus has Z = 11 throughout. Unusual neutron/charge settings are hypothetical bookkeeping, not claims of isotope or ion stability. The diagram encodes counts, not positions or sizes.
Read the picture in three steps
- Identify the chemical species and the quantities each label or axis represents. Read the units and any scale assumptions before comparing values.
- Na with A=23, q=1: 11 protons, 12 neutrons, 10 electrons.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Protons and neutrons form the nucleus; electrons occupy the space around it. A scale drawing would make the nucleus far smaller than the whole atom, so our bookkeeping graphic is not a size diagram.
Use protons = Z, neutrons = A−Z and electrons = Z−q, with signed q. A negative q increases the electron count. Ordinary ion formation changes electrons, not protons.
Mass number is an integer nucleon count for an isotope, whereas average atomic mass is a weighted measured mass. Neither alone tells you an ion’s electron count without charge information.
A worked example, step by step
Find protons, neutrons and electrons in ²⁴Mg²⁺, with Z = 12.
- Proton count is Z = 12.
- Neutrons = A−Z = 24−12 = 12.
- Electrons = Z−q = 12−(+2) = 10.
- The missing two electrons explain the +2 charge; the nucleus remains magnesium.
A positive ion has lost electrons; changing proton number changes the element.
What changes between neutral ²³Na and ²⁴Na?
Compare with an explanation
One neutron; each has 11 protons and 11 electrons.
Predict. Change one thing. Explain.
Use the sodium bookkeeping model. Change neutron count and charge independently; identify which change affects mass number and which affects electron count. Unusual settings are hypothetical counts, not stability predictions.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Na with A=23, q=1: 11 protons, 12 neutrons, 10 electrons.
Sodium nucleus has Z = 11 throughout. Unusual neutron/charge settings are hypothetical bookkeeping, not claims of isotope or ion stability. The diagram encodes counts, not positions or sizes.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use particle counts, mass or charge balance, electron structure, or nuclear attraction to justify your prediction. Separate an observation from an explanation.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionFor ³⁵Cl⁻ with Z=17, calculate all three subatomic counts. Compare it with neutral ³⁷Cl and explain which is an isotope change and which is an ion change.
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Compare with the answer and four-point rubric
- 1 point: ³⁵Cl⁻ has 17 protons and 18 neutrons.
- 1 point: It has 18 electrons because q = −1.
- 1 point: Neutral ³⁷Cl has 17 protons, 20 neutrons and 17 electrons.
- 1 point: The mass-number difference reflects neutrons; the charge difference reflects electrons. Both remain chlorine.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Which count defines the element?
Protons.
RECALL 2Which count usually changes in ion formation?
Electrons.
RECALL 3Is mass number the same as average atomic mass?
No: nucleon count versus isotope-weighted mass.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Count protons, neutrons and electrons separately
- p = Z.
- n(neutrons) = A−Z.
- e = Z−q, with signed charge q.
Remember: A positive ion has lost electrons; changing proton number changes the element.
Conditions: Sodium nucleus has Z = 11 throughout. Unusual neutron/charge settings are hypothetical bookkeeping, not claims of isotope or ion stability. The diagram encodes counts, not positions or sizes.
Refresh Kid · AP Chemistry Unit 1 · Objectives 1.5.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 1.5, objectives 1.5.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 1: Atomic Structure and Properties, Topics 1.1–1.8. The topic mapping identifies a framework area; focused lesson titles are our own teaching sequence. Molecular-formula scaling is an application of empirical composition. Models explicitly distinguish atom counts, molecule counts, mass fractions and electron structure. Spectra marked schematic are not measured data. Mass spectra here use single-element, singly charged monatomic ions. Configurations avoid Aufbau exceptions and individual quantum-number assignments. Qualitative attraction and size indices are not exact atomic predictions. The optional NaCl-type spatial block supplements complete charge-balance explanations. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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