Change electrons without changing the nucleus
You will be able to: Write common ion configurations, including removal of 4s electrons before 3d in Fe ions.
How does an ion’s configuration differ from the neutral atom?
Neutral sodium has one electron beyond the neon core. Removing it forms Na⁺ with the same electron count as neon, but Na⁺ still has 11 protons and is not a neon atom.
A useful starting point: Fill equal-energy orbitals before pairing electrons →
Words and symbols before equations
- Cation
- Positively charged species with fewer electrons than protons.
- Anion
- Negatively charged species with more electrons than protons.
- Isoelectronic
- Having the same number of electrons; it does not mean identical size or element.
What this picture assumes
Cases: 1 Na⁺, 2 Mg²⁺, 3 Al³⁺, 4 F⁻, 5 O²⁻, 6 Fe²⁺, 7 Fe³⁺. Configurations are ground-state examples. Fe ions lose 4s electrons before 3d.
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.
- Mg²⁺: 12 protons and 10 electrons; [Ne].
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Determine the electron count from Z−q first. Main-group cations lose outer-shell electrons; anions gain electrons into the next available positions.
Na⁺, Mg²⁺, Al³⁺, F⁻ and O²⁻ all have ten electrons and configuration [Ne]. Their nuclear charges differ, so their attraction and sizes differ.
Transition-metal ion formation needs care: Fe is [Ar]3d⁶4s², Fe²⁺ is [Ar]3d⁶ and Fe³⁺ is [Ar]3d⁵. Remove 4s electrons before 3d for these ions. Ion formation is not simply reversing the order in which a neutral-atom configuration was written.
A worked example, step by step
Write configurations for Mg²⁺ (Z=12) and Fe³⁺ (Z=26).
- Mg²⁺ has 12−2=10 electrons.
- Remove Mg’s two 3s electrons: Mg²⁺ is [Ne].
- For Fe³⁺, start at [Ar]3d⁶4s² and remove both 4s electrons, then one 3d electron.
- Fe³⁺ is [Ar]3d⁵; 18+5=23 equals 26−3.
An ion with a noble-gas electron count is not the noble-gas element; its proton count is unchanged.
Does Fe²⁺ retain 4s² because it filled before 3d?
Compare with an explanation
No. Both 4s electrons are removed in forming Fe²⁺.
Predict. Change one thing. Explain.
Select a supplied ion case. Verify its charge by subtracting electron count from proton count, then compare the neutral and ion configurations.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Mg²⁺: 12 protons and 10 electrons; [Ne].
Cases: 1 Na⁺, 2 Mg²⁺, 3 Al³⁺, 4 F⁻, 5 O²⁻, 6 Fe²⁺, 7 Fe³⁺. Configurations are ground-state examples. Fe ions lose 4s electrons before 3d.
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 questionCompare F⁻ (Z=9) and Na⁺ (Z=11): electron count, configuration, proton count and whether they are the same element.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: F⁻ has 10 electrons; Na⁺ has 10 electrons.
- 1 point: Both have 1s²2s²2p⁶.
- 1 point: Their proton counts are 9 and 11 respectively.
- 1 point: They are isoelectronic, not identical elements; nuclear charge differs.
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 1What stays fixed during ion formation?
The nucleus and element identity.
RECALL 2What does isoelectronic mean?
Equal electron count.
RECALL 3Which electrons leave first for Fe²⁺?
The two 4s electrons.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Change electrons without changing the nucleus
- Electrons = Z−q.
- For Fe cations, remove 4s electrons before 3d.
Remember: An ion with a noble-gas electron count is not the noble-gas element; its proton count is unchanged.
Conditions: Cases: 1 Na⁺, 2 Mg²⁺, 3 Al³⁺, 4 F⁻, 5 O²⁻, 6 Fe²⁺, 7 Fe³⁺. Configurations are ground-state examples. Fe ions lose 4s electrons before 3d.
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.
Want to work through this with a tutor?
Bring your question about Change electrons without changing the nucleus. Your explanation and answers remain free to access.