Did you know? The 4s subshell is commonly filled before 3d when building a neutral atom’s school-level configuration, yet when a first-row transition metal forms a positive ion its 4s electrons are generally removed first. Filling order and removal order are not the same rule.
Many students can write the electron configuration of iron correctly and still write Fe²⁺ incorrectly. The reliable approach is to count the electrons, record the neutral configuration, apply the ion’s charge, and remove electrons from the appropriate occupied subshell. This Hougang JC1 H2 Chemistry guide compares Fe, Cr and Cu while avoiding the misleading idea that a simplistic orbital ladder works unchanged for every ion.
This original Hougang H2 Chemistry tutor guide is aligned to the Singapore–Cambridge 2027 H2 Chemistry syllabus 9476. It uses hypothetical data and paper-based reasoning, not official examination questions or reports of observed student performance. Match the content to the actual examination year and school teaching sequence.
EDUKATESG · HOUGANG CHEMISTRY · TUTORIAL GUIDE
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Ten chapters · Worked evidence · Tutor checklist · Parent and student FAQs · Syllabus sources
Full chapter index
- 1. Start with the electron count, not an orbital mnemonic
- 2. Write the conventional neutral iron configuration
- 3. Understand why removal begins with 4s for these ions
- 4. Derive Fe²⁺ explicitly
- 5. Move from Fe²⁺ to Fe³⁺ without restoring 4s
- 6. Treat chromium’s neutral atom as a specific exception
- 7. Handle copper without treating its 3d count as nine
- 8. Separate d-block classification from an ionic electron count
- 9. Diagnose plausible wrong answers with two independent audits
- 10. Transfer the rule to another first-row cation
1. Start with the electron count, not an orbital mnemonic
Every neutral atom has as many electrons as protons. Iron has proton number 26 and therefore 26 electrons as a neutral atom; Fe²⁺ has 24 electrons and Fe³⁺ has 23. Copper has proton number 29, so Cu²⁺ contains 27 electrons. Counting first creates an audit that the final orbital notation must pass.
The periodic table helps identify the neutral element and how its valence orbitals are normally represented, but ionic charge must be applied after the neutral inventory is established. Students often start copying a familiar 3d count without checking the requested oxidation state. That can produce a neat but chemically inconsistent answer.
For these examples we use noble-gas shorthand [Ar], representing 18 core electrons. The remaining electrons are written explicitly in the 3d and 4s subshells. Square brackets around Ar do not mean an ion charge; they denote a filled argon-like core.
Student prompt: How many electrons do Fe, Fe²⁺ and Fe³⁺ contain, and why are their numbers different?
What the tutor should check: The learner gives 26, 24 and 23, accounting for each positive charge.
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2. Write the conventional neutral iron configuration
The accepted school-level ground-state configuration for neutral iron is [Ar] 3d⁶ 4s². Argon contributes 18 electrons; the 3d subshell contributes six and the 4s subshell contributes two. Their total is 26. An alternative notation orders occupied subshells as [Ar] 4s² 3d⁶, but the electron count and identity are the same.
Students learn an approximate orbital-filling order where 4s is populated before 3d while moving through neutral atoms. This mnemonic is useful, but it must not be turned into an unchanging energy ordering for all charge states. Orbital energies shift with electron occupancy and nuclear attraction in a many-electron atom or ion.
Ask students to point to the occupied 4s electrons before considering ion formation. Then write a neutral-ion transition with the electron inventory explicitly shown. This is a more accurate teaching step than saying ‘erase any two electrons anywhere’.
Student prompt: Check the electron total represented by [Ar] 3d⁶ 4s² and explain the roles of the bracketed core and outer subshells.
What the tutor should check: The student correctly adds 18 + 6 + 2 and identifies the two 4s electrons.
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3. Understand why removal begins with 4s for these ions
For the first-row transition-metal cations in this syllabus, the electrons occupying 4s are generally removed before those occupying 3d. This is consistent with the change in relative orbital energies and the outer-shell character of 4s in the ionisation process. The simplified filling chart used for neutral atoms is not a literal universal ionisation sequence.
Do not claim that electrons fill ‘in reverse alphabetical order’ or that 3d is always higher in energy than 4s under every circumstance. Energy levels depend on the species and electron population. The exam-safe method is to write the known neutral configuration, remove 4s electrons first, and then remove 3d electrons if the ion’s charge requires more.
A paper question that changes Fe²⁺ to Fe³⁺ is particularly revealing: once the two 4s electrons have been removed, the third electron comes from 3d. The rule should follow the species’ current occupancy rather than a memorised list of shell names.
Student prompt: When iron forms Fe²⁺, which occupied subshell loses two electrons first, and why does the neutral filling mnemonic not settle the question?
What the tutor should check: The learner removes the 4s pair and explains that ion electron configurations follow the charged species, not simply the neutral Aufbau diagram.
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4. Derive Fe²⁺ explicitly
Begin with neutral Fe: [Ar] 3d⁶ 4s². Removing two electrons from 4s produces Fe²⁺: [Ar] 3d⁶. The 18 core electrons plus six d electrons total 24. This is the electron count expected from proton number 26 and charge +2.
A common incorrect answer is [Ar] 3d⁴ 4s². It has the right total number of electrons, but it removes them from the wrong subshell and retains the 4s pair. Correct electron totals are necessary, not sufficient. The distribution must also fit the appropriate ionic electronic structure.
Ask the learner to compare the two options. The incorrect configuration can look plausible because it includes 24 electrons, so a tutor should require both a total-electron audit and a subshell-removal explanation. A correct final string without a reason is less transferable.
Student prompt: Write Fe²⁺ in noble-gas notation and identify the tempting but incorrect [Ar] 3d⁴ 4s² option.
What the tutor should check: The student chooses [Ar] 3d⁶ and explains why removing 3d electrons first was wrong.
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5. Move from Fe²⁺ to Fe³⁺ without restoring 4s
Fe³⁺ has three fewer electrons than the neutral atom. Remove the two 4s electrons from [Ar] 3d⁶ 4s², then one 3d electron. The result is [Ar] 3d⁵, with 18 + 5 = 23 electrons. The sequence makes the charge change visible.
Some students write [Ar] 3d³ 4s² because they assume removing a third electron means continue removing from d after two d removals. Others write [Ar] 3d⁶ 4s⁻¹, which is not a sensible occupied-subshell notation. Both errors come from not recording the correct first step.
In an ideal isolated-ion orbital-filling discussion, d⁵ is a special half-filled d-subshell occupancy, but a student should avoid claiming that every Fe³⁺ complex must have exactly five unpaired electrons. Ligand-field splitting and spin-state questions depend on the surroundings and require additional information.
Student prompt: Starting from neutral Fe, remove three electrons in the correct order and check Fe³⁺ against its total electron count.
What the tutor should check: They obtain [Ar] 3d⁵ and can name each removed subshell.
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6. Treat chromium’s neutral atom as a specific exception
Chromium, proton number 24, has the observed neutral ground-state configuration [Ar] 3d⁵ 4s¹ rather than the naive [Ar] 3d⁴ 4s² filling prediction. The 3d⁵ 4s¹ pattern is commonly taught as a stabilised arrangement, but real orbital energies involve many-electron effects rather than a single absolute ‘half-filled rule’ for every atom.
For Cr³⁺, remove the one occupied 4s electron and then two 3d electrons. This gives [Ar] 3d³, with 18 + 3 = 21 electrons; chromium has 24 protons and a charge of +3, so the total is consistent.
Teach the neutral exception before ionisation. If students begin from the naive chromium configuration, they may still occasionally reach an apparently familiar ion count, but their reasoning about the actual ground-state atom is incorrect and will break on other examples.
Student prompt: Explain why Cr³⁺ is [Ar] 3d³ when the neutral chromium configuration is [Ar] 3d⁵ 4s¹.
What the tutor should check: The learner removes one 4s and two 3d electrons and verifies 21 remaining.
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7. Handle copper without treating its 3d count as nine
Copper, proton number 29, has neutral ground-state configuration [Ar] 3d¹⁰ 4s¹. This is another common departure from the simplest neutral filling mnemonic. For Cu⁺, remove the 4s electron to obtain [Ar] 3d¹⁰. For Cu²⁺, remove one additional 3d electron to obtain [Ar] 3d⁹.
The counts check: Cu⁺ has 28 electrons, or 18 + 10. Cu²⁺ has 27 electrons, or 18 + 9. A student who writes Cu²⁺ as [Ar] 3d¹⁰ 4s⁻¹ has tried to remove an electron from a subshell after it is empty; the bookkeeping should stop at the actual occupied electrons.
Connect the result to later transition-metal chemistry without assuming every complex of copper has a particular colour or geometry from the bare-ion configuration alone. The d-electron count is one input to ligand-field reasoning; the coordination environment matters as well.
Student prompt: Give Cu, Cu⁺ and Cu²⁺ ground-state electron configurations and verify each electron total.
What the tutor should check: The learner writes [Ar]3d¹⁰4s¹, [Ar]3d¹⁰ and [Ar]3d⁹.
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8. Separate d-block classification from an ionic electron count
The d-block refers to elements whose relevant differentiating electrons occupy d orbitals, while a transition element is defined in the 2027 H2 syllabus as a d-block element whose atom has an incomplete d subshell or can form a cation with an incomplete d subshell. Students should use the exact definition rather than simply naming all d-block elements as identical.
Iron forms Fe²⁺ and Fe³⁺ with incomplete d subshells; copper can form Cu²⁺ with d⁹. By contrast, the monovalent Cu⁺ ion has d¹⁰. The existence of a d¹⁰ ion does not make every oxidation state of copper a ‘non-transition element’; element classification concerns the defined atomic or ionic possibilities.
Zinc commonly forms Zn²⁺ with a full d¹⁰ subshell, and neutral Zn has 3d¹⁰ 4s². In the school definition, zinc is a useful boundary example: a d-block element not counted as a transition element under that criterion. The answer should explain why, rather than appeal only to periodic-table position.
Student prompt: Why can copper count as a transition element even though Cu⁺ has a full 3d subshell?
What the tutor should check: The learner uses the possible Cu²⁺ d⁹ ion and the syllabus definition.
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9. Diagnose plausible wrong answers with two independent audits
An electron configuration should pass an electron-count test and an occupied-subshell test. A student can get one right and the other wrong. For example, the spurious Fe²⁺ string [Ar] 3d⁴ 4s² totals 24 electrons but contradicts the expected removal of 4s electrons during first-row transition-metal cation formation.
Another frequent issue is mixing the notational order of written subshells with the order in which electrons leave an ion. Writing ‘[Ar] 3d⁶ 4s²’ rather than ‘[Ar] 4s² 3d⁶’ does not imply different chemistry; those strings list the same occupancy. However, a removal arrow must act on a real occupied orbital.
Feedback should therefore name the species and charge, require an electron inventory, then ask the learner which occupied subshell loses the next electron. Treat an incorrect Cr or Cu ground-state starting point separately from an incorrect ionic-removal rule.
Student prompt: A learner produces [Ar] 3d⁴ 4s² for Fe²⁺. Which audit passes, and which fails?
What the tutor should check: The learner recognises the correct total 24 but wrong removal sequence and orbital occupancy.
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10. Transfer the rule to another first-row cation
Use a fresh example: neutral nickel has proton number 28 and is represented in the school model as [Ar] 3d⁸ 4s². For Ni²⁺, remove the two 4s electrons to obtain [Ar] 3d⁸, with 26 electrons. For Ni³⁺, if discussed as a symbolic cation, a further 3d electron would be removed, giving [Ar] 3d⁷.
This is a paper electronic-configuration exercise, not a claim that every high oxidation state has the same chemical stability or can be isolated under all conditions. Real ions in complexes or solids may require a deeper orbital and bonding description; the simple electron inventory remains a useful first check.
Parents can ask their child whether the 4s electrons are written first in a list and whether that implies they must be removed last. A correct response identifies the difference between neutral filling conventions and cation ionisation, then demonstrates it on a new element rather than repeating Fe²⁺.
Student prompt: Write Ni²⁺ from [Ar] 3d⁸ 4s² without skipping the electron-count check.
What the tutor should check: The learner gives [Ar] 3d⁸ and confirms that 28 − 2 = 26 electrons remain.
Worked evidence: the full comparison
Each row uses the conventional first-row atom/ion configurations taught in H2 Chemistry; the [Ar] core stands for 18 electrons. ‘Remove 4s first’ is a rule for the relevant first-row transition-metal cations, not a universal claim about orbital energies.
| Species | Electron count | Ground-state configuration / reasoning |
|---|---|---|
| Neutral Fe | 26 | [Ar] 3d⁶ 4s² |
| Fe²⁺ | 24 | [Ar] 3d⁶ (remove both 4s electrons) |
| Fe³⁺ | 23 | [Ar] 3d⁵ (then one 3d electron) |
| Neutral Cr | 24 | [Ar] 3d⁵ 4s¹ (neutral exception) |
| Cr³⁺ | 21 | [Ar] 3d³ (remove one 4s and two 3d) |
| Neutral Cu | 29 | [Ar] 3d¹⁰ 4s¹ (neutral exception) |
| Cu⁺ | 28 | [Ar] 3d¹⁰ |
| Cu²⁺ | 27 | [Ar] 3d⁹ |
| Neutral Ni → Ni²⁺ | 28 → 26 | [Ar] 3d⁸ 4s² → [Ar] 3d⁸ |
| Check | Z minus positive charge | Electron count AND subshell removal must both agree |
Fe²⁺, Fe³⁺, Cr³⁺ and Cu²⁺ show how an ion’s d-electron count depends on its neutral configuration and the charge. A neat electron total by itself cannot verify correct occupied orbitals.
The tutor’s preparation checklist
- Confirm the 2027 H2 Chemistry 9476 sections on atomic structure and transition elements and the student’s own examination year.
- Start with Z and calculate the electron count for every requested positive ion.
- Write the correct neutral configuration, including the Cr and Cu exceptions when relevant.
- Remove occupied 4s electrons before 3d electrons for the first-row transition-metal cations in these examples.
- Audit both electron count and the resulting 3d occupancy; do not substitute an inappropriate bare-ion spin claim for a ligand-field analysis.
- Finish with a changed ion such as Ni²⁺, checking whether the learner explains the order without reading the model solution.
At the end of a small-group lesson, the next prompt should alter one important detail while keeping the method recognisable. A child should explain what stays fixed, what changes and what new information is needed. When an answer is wrong, identify the first mistaken chemical assumption before practising the same arithmetic again.
Frequently asked questions
Why is 4s filled before 3d but removed first?
The familiar neutral-atom filling sequence is a useful approximation; orbital energies and occupations change in transition-metal cations. For these cations the occupied 4s electrons are generally removed before 3d electrons.
What is the electron configuration of Fe²⁺?
It is [Ar] 3d⁶, formed by removing iron’s two 4s electrons from neutral [Ar] 3d⁶ 4s².
What is the configuration of Fe³⁺?
It is [Ar] 3d⁵. After removing two 4s electrons from iron, the next electron is removed from the 3d subshell.
Why is chromium an exception?
Neutral chromium’s accepted ground-state configuration is [Ar] 3d⁵ 4s¹ rather than the simplistic [Ar] 3d⁴ 4s² prediction. The actual ordering reflects many-electron energies.
Why is Cu²⁺ 3d⁹ rather than 3d¹⁰?
Neutral copper is [Ar] 3d¹⁰ 4s¹. Removing one 4s and one 3d electron to form Cu²⁺ leaves [Ar] 3d⁹.
Does a d-electron configuration alone determine a complex ion’s colour and spin?
No. It is an input to a bonding explanation, but ligand type, geometry and orbital splitting also matter. Avoid predicting a particular complex’s magnetic behaviour from the isolated ionic count alone.
Next reading: connected teaching guides
- JC1 H2 Chemistry Tutor | Successive ionisation and shell inference
- JC2 H2 Chemistry Tutor | Copper ligand exchange and colour changes
- Hougang Chemistry Tuition and Tutor Library
For a discussion about support, bring the child’s H1 or H2 course, examination year and one current piece of schoolwork that shows the problem. Confirm actual tutorial availability, group size, programme and venue directly with the provider. This article does not guarantee enrolment or examination results.
Sources, syllabus fit and limits
- SEAB: 2027 H2 Chemistry syllabus (9476)
- SEAB: 2027 A-Level school-candidate syllabus listings
- Hougang Chemistry Tuition and Tutor Library
- eduKateSG: immutable Secondary 1 Mathematics Clementi small-group tutorial reference
The official 2027 H2 Chemistry 9476 syllabus covers configurations of atoms and ions in Atomic Structure and specifically first-row transition elements and their ions in Section 13. This is a teaching model for those electronic inventories. The energy-order explanation is qualified rather than presented as a universal orbital rule applying to every chemical environment.
All example numbers, diagrams, prompts and comparisons were prepared as original educational illustrations. They are not official mark schemes, observations from a laboratory or testimonials. Students should use their applicable official syllabus and school materials and undertake any practical Chemistry activity only under suitable supervision. Properly taught students should be able to defend a result with evidence and recognise its limits.