Why does iron lose its 4s electrons before its 3d electrons when it forms Fe²⁺? This is one of the most useful H2 Chemistry atomic-structure questions because it exposes a misunderstanding that can follow a JC student into transition-metal complexes, oxidation states and unfamiliar data-based questions. The short answer: the 4s subshell generally fills before 3d in the early neutral-atom building sequence, but electrons are removed from the 4s subshell first when the relevant transition-metal atom forms a positive ion. Filling order is not a permanent removal order.
If you are looking for an H2 Chemistry tutor in Hougang, JC1 atomic structure help, JC2 transition metal electron configuration tuition, or Singapore A-Level Chemistry 9476 worked examples, I would begin with a small test: write neutral Fe, Fe²⁺ and Fe³⁺ without looking at notes, then explain each change. Getting the final notation right by chance is less important than showing why the ion is not written with the old 4s electrons still attached.
This guide teaches a repeatable way to answer transition-metal-ion electronic configuration questions, recognises common Cr and Cu exceptions, and shows how to check your work. It is for students moving from JC1 into JC2; check your school’s exact examination-year syllabus because H2 Chemistry syllabuses can differ across cohorts.
Start at the Hougang Chemistry learning library · Check the official SEAB A-Level syllabus directory
The first trap: confusing the order of filling with the order of removal
You may have learnt the useful Aufbau-style sequence 1s, 2s, 2p, 3s, 3p, 4s, 3d. This is a practical first model for many neutral atoms, not a universal statement that 4s must always lie below every 3d state in every atom or ion. Orbital energies depend on electron occupancy, shielding, nuclear attraction and interactions between electrons.
As the 3d subshell becomes occupied, the relative energies and radial distributions of 4s and 3d electrons change. In the familiar first-row transition-metal cations, the electrons conventionally assigned to 4s in the neutral atom are removed before 3d electrons. You do not need to imagine little labelled balls travelling between fixed shelves. The electronic configuration represents the electronic state of the species that exists now, not a historical log of which electron arrived first.
The most helpful exam habit is to keep two operations separate: first write the neutral atom correctly; next remove the stated number of electrons in the correct order; finally check the total electron count and ionic charge.
Worked example 1: iron, Fe²⁺ and Fe³⁺
Iron has proton number 26. A neutral atom has 26 electrons. The ground-state configuration commonly written at H2 level is Fe: [Ar] 3d⁶ 4s². The notation [Ar] represents the 18-electron argon core, and 6 + 2 electrons lie beyond it. Some books place 4s before 3d when listing subshells; neither typography nor the order on the line changes the actual occupancy.
- Make Fe²⁺. Remove two electrons from 4s, leaving [Ar] 3d⁶. Count: 18 + 6 = 24 electrons, appropriate for an ion with 26 protons and charge +2.
- Make Fe³⁺. Remove one more electron, now from 3d, leaving [Ar] 3d⁵. Count: 18 + 5 = 23 electrons; the ionic charge is +3.
- Reverse-check. Fe²⁺ → Fe³⁺ + e⁻ is an oxidation half-equation; the Fe ion loses one electron and its charge increases by one positive unit.
- Explain, do not chant. The 4s electrons are lost first on cation formation; after 4s is emptied, further removal is from 3d.
A common incorrect answer is Fe²⁺: [Ar] 3d⁴ 4s². It subtracts two electrons from 3d simply because 3d appears last in the student’s written neutral configuration. The electron count may still equal 24, which shows why counting alone is a necessary but insufficient check.
A compact reference table for the most useful examples
| Species | Neutral starting configuration | Cation result | Reasoning checkpoint |
|---|---|---|---|
| Fe / Fe²⁺ | [Ar] 3d⁶ 4s² | Fe²⁺: [Ar] 3d⁶ | Two 4s electrons removed |
| Fe / Fe³⁺ | [Ar] 3d⁶ 4s² | Fe³⁺: [Ar] 3d⁵ | Two 4s then one 3d removed |
| Cr / Cr³⁺ | [Ar] 3d⁵ 4s¹ | Cr³⁺: [Ar] 3d³ | One 4s then two 3d removed |
| Cu / Cu²⁺ | [Ar] 3d¹⁰ 4s¹ | Cu²⁺: [Ar] 3d⁹ | One 4s then one 3d removed |
| Mn / Mn²⁺ | [Ar] 3d⁵ 4s² | Mn²⁺: [Ar] 3d⁵ | Two 4s electrons removed |
These are familiar isolated-ion configurations at this teaching level. Be cautious when extending the same simple notation to real complexes, where electron arrangements, spin states and ligand-field effects require additional information. The overall idea stays useful; the details become richer.
Why chromium and copper make memorisation dangerous
A student who blindly applies the simplest orbital-filling chart might predict Cr as [Ar] 3d⁴ 4s² and Cu as [Ar] 3d⁹ 4s². The familiar observed ground states are instead Cr: [Ar] 3d⁵ 4s¹ and Cu: [Ar] 3d¹⁰ 4s¹. In these cases, the detailed energy balance favours the displayed arrangements. It is tempting to say ‘half-filled and filled subshells are always stable’; this slogan is only a shorthand, not a general predictive law for every element.
For Cr³⁺, start from Cr 3d⁵ 4s¹, remove the single 4s electron, then two 3d electrons. For Cu²⁺, start from 3d¹⁰ 4s¹, remove one 4s electron and one 3d electron. Do not first rewrite the unusual neutral atom as the convenient but incorrect default state.
How to diagnose the first wrong thought
| What the student writes | Likely underlying issue | Repair question |
|---|---|---|
| Fe²⁺ = 3d⁴ 4s² | Removed from 3d before 4s | Which subshell is emptied first when the ion forms? |
| Cr³⁺ = 3d¹ 4s² | Used a default neutral Cr configuration, then removed from 3d | What is neutral chromium’s actual ground state? |
| Fe³⁺ contains 26 electrons | Confused proton number with electron count in an ion | How does a +3 charge change the electron total? |
| A correct configuration but no explanation | Recalled symbols without a model | Can you justify the last electron removed? |
| All d⁶ complexes behave identically | Transferred an isolated-ion configuration into ligand-field predictions | What additional information about ligands and geometry would matter? |
When I read a corrected answer, I look for evidence that the student can handle a changed ion, not merely reproduce the iron example. A small test using Mn²⁺ and Cr³⁺ often reveals whether the rule has genuinely transferred.
Worked example 2: use the electron count to expose an impossible answer
Suppose someone writes Cu²⁺ as [Ar] 3d¹⁰ 4s⁻¹. A negative number of electrons in a subshell is not a physical occupancy. Return to neutral Cu, 18 + 10 + 1 = 29 electrons. Cu²⁺ has 27 electrons. Remove the one 4s electron and one 3d electron: 18 + 9 = 27. The correct configuration is [Ar] 3d⁹.
Now try Co³⁺. Neutral cobalt (Z = 27) is commonly [Ar] 3d⁷ 4s². Remove two 4s electrons and one 3d electron: Co³⁺ becomes [Ar] 3d⁶, with 24 electrons. Notice that Fe²⁺ and Co³⁺ can share the same number of d electrons, even though they are different elements and ions. This is one reason electronic configuration is a starting descriptor, not a full chemical identity.
What an H2 examiner can do with this foundation
- Electronic configuration: ask for a transition-metal cation rather than its neutral atom.
- Successive ionisation and period trends: ask you to reason about nuclear attraction, shielding, electron pairing and changing orbital occupancy, not just recite a rule.
- Oxidation states: connect loss or formal assignment of electrons with symbols, charge and redox bookkeeping.
- Complex-ion reasoning: introduce additional information about ligands and coordination chemistry, where electronic structure influences observable properties.
- Data-based questions: provide an unfamiliar ion and require consistency between electron count, oxidation state and a measured property.
Not every extension about molecular orbitals or detailed ligand-field theory is a compulsory H2 learning outcome. A good tutor should mark clearly where syllabus content ends and university-level explanation begins, especially for students who are keen to explore further.
Independent diagnostic: do these without your notes
- Write the ground-state configuration of neutral Mn (Z = 25), then Mn²⁺.
- Write the ground-state configuration of neutral Cr (Z = 24), then Cr³⁺.
- Write the ground-state configuration of neutral Cu (Z = 29), then Cu⁺.
- Explain in one sentence why Fe²⁺ and Fe³⁺ have different numbers of 3d electrons.
- Find the error in the claim: ‘The last subshell filled must always be the first subshell emptied.’
- Decide whether an electron count check alone is enough to prove your configuration correct.
Answers and how to reason
1. Mn: [Ar] 3d⁵ 4s²; Mn²⁺: [Ar] 3d⁵. 2. Cr: [Ar] 3d⁵ 4s¹; Cr³⁺: [Ar] 3d³. 3. Cu: [Ar] 3d¹⁰ 4s¹; Cu⁺: [Ar] 3d¹⁰. 4. Fe³⁺ has lost one more 3d electron than Fe²⁺. 5. Filling sequence cannot substitute for the actual ordering on ion formation. 6. No: a student can count the right total while assigning those electrons to the wrong subshells.
A one-week study route that fits a busy JC timetable
- Day 1: write the first-row transition-metal neutral configurations that appear in your school notes; identify the exceptions.
- Day 2: practise 2+ and 3+ ions with no examples on the page.
- Day 3: correct three deliberately wrong configurations, naming the first faulty step.
- Day 4: connect one ion to a redox half-equation and explain its electron loss.
- Day 5: solve an unfamiliar mixed question from a school tutorial or approved practice set.
- Day 6: compare your answer with official or school marking guidance and correct language, not only symbols.
- Day 7: repeat the original diagnostic from memory, then explain the rule to someone else in under a minute.
If your school assessment is very close, compress the routine into short sessions and prioritise your teacher’s assigned questions. The important thing is the feedback loop: attempt, identify the specific fault, repair the model, and verify it on a new example.
What parents in Hougang can ask an H2 Chemistry tutor
Bring one piece of marked JC1 or JC2 Chemistry work and ask the tutor to diagnose it while the student explains aloud. Is the problem a missing neutral-atom exception, incorrect 4s removal, poor electron counting, or a broader confusion over charge and oxidation state? A productive small-group lesson makes every learner complete an independent version before hearing other answers; a private lesson can do the same diagnostic without the group element.
Ask about the tutor’s actual H2 syllabus experience, availability, lesson venue and travel arrangements rather than assuming that a Hougang-targeted page guarantees a class physically located in Hougang. Improvement should be visible in changed questions, not only in the number of notes printed.
How this small rule grows into university science
Electronic configuration later helps make sense of catalysts, coordination compounds, magnetic behaviour, materials and redox processes. Chemistry, pharmacy, materials science, chemical engineering and related courses study these ideas at different levels. No single H2 grade promises entry to a particular programme; admission requirements and course choices must be checked with the universities. What remains valuable beyond any exam is the habit of asking whether a model still works when the chemical species changes.
Common questions
Do electrons fill 4s before 3d?
The usual early neutral-atom construction model fills 4s before 3d, with important exceptions and energy subtleties. Do not treat that construction as the correct order for removing electrons from first-row transition-metal atoms.
Why is Fe³⁺ written [Ar] 3d⁵?
Neutral Fe is [Ar] 3d⁶ 4s². Removing three electrons takes the two 4s electrons and one 3d electron, leaving [Ar] 3d⁵.
Should I learn the quantum mechanical derivation for A-Level H2?
Understand that occupancy and orbital energies change and follow your exact syllabus and teacher’s required depth. A university-level orbital-energy calculation is not a substitute for the examined electronic configurations.
Can good grades come from memorising the configurations alone?
Memorising a few examples helps recall, but transfer depends on explaining the rule, counting electrons and handling an unfamiliar ion correctly.
Continue learning: JC1 H2 atomic structure and periodic trends · Why first-ionisation-energy trends have exceptions · Hougang Chemistry Tutor and Tuition Library · Official SEAB syllabus information. For current tuition arrangements, contact eduKate Singapore.