Aufbau: orbitals filled in order of increasing energy (1s<2s<2p<3s<3p<4s<3d<...). Pauli: no two electrons in same atom have identical 4 quantum numbers. Hund: orbitals of same energy first filled singly with parallel spins.
-- NCERT Class 11 Chemistry, Ch. 2, p. 62Electronic Configuration
8 MCQs9-step worked example
Source: NCERT Structure of AtomPYQ coverage: NEET 2021Official key: NTA-verifiedLast updated: 27 Sep 2026
Electronic Configuration, explained for NEET
The configuration of Cr is not [Ar] 3d⁴ 4s². It is [Ar] 3d⁵ 4s¹. If you wrote the former in a NEET paper, you lost a mark and gained a negative — that is the trap this lesson exists to fix.
Electronic configuration is the distribution of electrons among available orbitals following three rules: Aufbau principle (fill lowest energy first), Pauli exclusion (no two electrons share all four quantum numbers), and Hund's rule (maximise spin multiplicity within a subshell). For most elements up to Z = 30, straightforward application of (n + l) ordering gives the correct ground-state configuration.
The exceptions that NEET tests repeatedly are chromium (Z = 24) and copper (Z = 29). The expected filling gives Cr: [Ar] 3d⁴ 4s² and Cu: [Ar] 3d⁹ 4s². The actual configurations are:
- Cr: [Ar] 3d⁵ 4s¹ — half-filled 3d achieves extra exchange-energy stability.
- Cu: [Ar] 3d¹⁰ 4s¹ — fully filled 3d achieves extra stability.
The underlying reason: exchange energy increases with the number of parallel-spin electron pairs. A half-filled or fully filled d-subshell maximises these pairs, and the energy gain from exchange exceeds the small 4s–3d gap for these specific elements.
NCERT Class 11 Chemistry Chapter 2, page 65, explicitly states this principle as part of the electronic configuration rules for transition elements.
Watch-out for ions: When forming Cr³⁺ or Cu²⁺, electrons are removed from 4s first (higher principal quantum number), then from 3d. Cr³⁺ is [Ar] 3d³, not [Ar] 3d¹ 4s². Cu²⁺ is [Ar] 3d⁹.
Can you answer these Electronic Configuration MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1CalculationPractice
Neutral iron (Fe, Z = 26) has ground-state configuration [Ar] 3d⁶ 4s². How many unpaired electrons are present in the Fe²⁺ ion?
Show answer and why every option is right or wrong
Answer: B. Fe²⁺ forms by removing electrons from the 4s orbital first, before any 3d electron, so both 4s electrons are lost and Fe²⁺ is [Ar] 3d⁶ (NCERT Class 12 Chemistry, Chapter 4, page 95: ns electrons are lost before (n − 1)d electrons). Distributing 6 electrons among the five 3d orbitals by Hund's rule, each orbital is first singly occupied (5 unpaired electrons); pairing begins only with the 6th electron, which must double up in an already-occupied orbital (NCERT Class 11 Chemistry, Chapter 2, page 63: pairing starts with the 6th electron in a d subshell). This leaves 4 orbitals singly occupied and 1 doubly occupied, so Fe²⁺ has 4 unpaired electrons.
Why A is wrong: A (6 unpaired) wrongly assumes all 6 d-electrons stay unpaired, ignoring that only 5 d-orbitals exist — the Pauli exclusion principle forces the 6th electron to pair up in an already-occupied orbital once all five orbitals hold one electron each.
Why C is wrong: C (0 unpaired) wrongly pairs the electrons two at a time from the start (filling 3 orbitals doubly, leaving 2 empty) instead of applying Hund's rule, which requires every orbital to be singly occupied before any pairing begins.
Why D is wrong: D (2 unpaired) comes from assuming a different d-electron count altogether, such as confusing Fe²⁺ (3d⁶) with an ion like Ni²⁺ (3d⁸, which has 2 unpaired electrons) rather than correctly identifying Fe²⁺ as 3d⁶.
MCQ 2Easy RecallPractice
What is the ground-state electronic configuration of neutral iron (Fe, Z = 26)?
Show answer and why every option is right or wrong
Answer: A. Fe (Z = 26) follows standard Aufbau filling — unlike Cr and Cu, it is not one of the two anomalies in the first transition series — giving [Ar] 3d⁶ 4s² (NCERT Class 11 Chemistry Chapter 2, page 64).
Why B is wrong: B wrongly applies a Cr/Cu-style 4s→3d shift to Fe, but Fe is not an exception — it follows plain Aufbau filling with 3d⁶ 4s².
Why C is wrong: C drops the 4s² electrons entirely, undercounting Fe's 26 electrons by two.
Why D is wrong: D places an electron in 4p before 3d and 4s are filled, violating the Aufbau order, and also gives the wrong total electron count for Z = 26.
MCQ 3Easy RecallPractice
Which of the following explains why Cr and Cu deviate from expected Aufbau filling?
Show answer and why every option is right or wrong
Answer: A. The additional exchange energy gained from maximising parallel-spin electron pairs in a half-filled (d⁵) or fully filled (d¹⁰) subshell provides extra stabilisation that outweighs the normal filling order (NCERT Class 11 Chemistry Chapter 2, page 65).
Why B is wrong: B is partially related to inter-electronic repulsion concepts but does not explain specifically why d⁵ and d¹⁰ are favoured — exchange energy is the accepted NCERT explanation.
Why C is wrong: C is incorrect — for Z ≤ 20 the 4s is actually lower than 3d; the relative ordering is Z-dependent and does not explain only Cr and Cu deviating.
Why D is wrong: D is fabricated — 4s is always available; shielding affects energy ordering, not orbital availability.
MCQ 4Direct ApplicationPractice
The electronic configuration of Cr³⁺ ion is:
Show answer and why every option is right or wrong
Answer: B. Cr is [Ar] 3d⁵ 4s¹. Removing 3 electrons: first remove 4s¹ (1 electron), then 3d (2 electrons), leaving [Ar] 3d³ (NCERT Class 12 Chemistry Chapter 4, page 95: when d-block elements form ions, ns electrons are lost before (n – 1)d electrons).
Why A is wrong: A retains 5 d-electrons, which means only the 4s¹ was removed — that accounts for only 1 electron removed, not 3.
Why C is wrong: C removes the right number of electrons (24 − 3 = 21 remain) but from the wrong orbitals: the 4s electron is lost first, so Cr³⁺ is [Ar] 3d³, not 3d² 4s¹.
Why D is wrong: D would require starting from [Ar] 3d⁴ 4s² (the wrong neutral config) and removing only 1 electron — uses the incorrect parent configuration and wrong removal count.
MCQ 5Direct ApplicationPractice
The electronic configuration of Cu²⁺ ion is:
Show answer and why every option is right or wrong
Answer: B. Cu is [Ar] 3d¹⁰ 4s¹. Remove 2 electrons: first 4s¹ (1 electron), then one from 3d, giving [Ar] 3d⁹. Electrons are always removed from the highest principal quantum number first.
Why A is wrong: A retains a 4s electron in the ion — for transition metal ions, 4s electrons are removed before 3d electrons, so 4s should be empty.
Why C is wrong: C removes zero 3d electrons (only removes 4s¹), which accounts for only 1 electron removed, not 2.
Why D is wrong: D starts from the wrong neutral configuration [Ar] 3d⁹ 4s² and removes both 4s electrons — uses incorrect parent configuration.
MCQ 6Direct ApplicationPractice
Among Fe (Z = 26), Co (Z = 27), Ni (Z = 28), and Cu (Z = 29), which has a ground-state configuration that deviates from simple Aufbau prediction?
Show answer and why every option is right or wrong
Answer: A. Among these four, only Cu (Z = 29) deviates: actual [Ar] 3d¹⁰ 4s¹ vs expected [Ar] 3d⁹ 4s². Fe, Co, and Ni follow standard Aufbau filling with [Ar] 3dⁿ 4s² (NCERT Class 11 Chemistry Chapter 2, page 64).
Why B is wrong: Co (Z = 27) is [Ar] 3d⁷ 4s² — standard Aufbau, no anomaly.
Why C is wrong: Fe (Z = 26) is [Ar] 3d⁶ 4s² — standard Aufbau, no anomaly.
Why D is wrong: Ni (Z = 28) is [Ar] 3d⁸ 4s² — standard Aufbau with no anomaly; only Cu and Cr in the first transition series show this deviation.
MCQ 7Concept TrapPractice
The number of unpaired electrons in Cr (Z = 24) in its ground state is:
Show answer and why every option is right or wrong
Answer: C. C is correct. Chromium is an exception to the aufbau order: its ground state is [Ar] 3d⁵ 4s¹, because a half-filled d subshell is especially stable. By Hund's rule the five 3d electrons sit one to an orbital, all unpaired, and the lone 4s electron is unpaired as well: 5 + 1 = 6.
Why A is wrong: A (4 unpaired) comes from the configuration [Ar] 3d⁴ 4s² — filling by the aufbau order without the chromium exception. That gives four singly occupied d-orbitals and a paired 4s².
Why B is wrong: B (5 unpaired) counts the 3d electrons and forgets that the single 4s electron is unpaired too.
Why D is wrong: D (3 unpaired) is the count for the Cr³⁺ ion, [Ar] 3d³, not for the neutral atom. The question asks about Cr in its ground state.
MCQ 8Easy RecallPractice
According to Hund's rule of maximum multiplicity, pairing of electrons in a d subshell begins with the entry of which electron into that subshell?
Show answer and why every option is right or wrong
Answer: B. Pairing does not start until every orbital of the subshell holds one electron. A d subshell has five orbitals, so the first five electrons go in singly and pairing begins with the 6th. NCERT states this directly: pairing starts in the p, d and f orbitals with the entry of the 4th, 6th and 8th electron respectively (NCERT Class 11 Chemistry, Chapter 2, pages 62–63).
Why A is wrong: A — the 4th electron is where pairing starts in a p subshell, which has only three orbitals.
Why C is wrong: C — the 8th electron is where pairing starts in an f subshell, which has seven orbitals.
Why D is wrong: D — 11 is one more than the d subshell's capacity of 10 electrons; it confuses the subshell's capacity with its number of orbitals.
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How do you solve a Electronic Configuration question? A worked example
Pattern: Electronic configuration writing with Cr/Cu anomaly (P.CHE.U02.AUFBAU_ELECTRONIC_CONFIG, observed NEET 2024)
- 1
Given
• Cr: atomic number 24• Cu⁺: atomic number 29, charge +1 (28 electrons)
- 2
Required
Ground-state electronic configuration and number of unpaired electrons for each species.
- 3
Concept
Electronic configuration follows Aufbau principle with exceptions for Cr and Cu due to extra exchange-energy stability of half-filled (d⁵) and fully filled (d¹⁰) subshells. For ions, electrons are removed from the highest principal quantum number (4s before 3d).
- 4
Formula/Rule
• Aufbau order: 1s < 2s < 2p < 3s < 3p < 4s < 3d...• Exception: Cr adopts 3d⁵ 4s¹; Cu adopts 3d¹⁰ 4s¹• Ion formation: remove from highest n first (4s before 3d)
- 5
Substitution
(a) Cr (24 electrons):• Expected: [Ar] 3d⁴ 4s² (18 + 4 + 2 = 24) ✗• Actual: [Ar] 3d⁵ 4s¹ (18 + 5 + 1 = 24) ✓ (half-filled d⁵ stability)
(b) Cu⁺ (28 electrons):• Neutral Cu: [Ar] 3d¹⁰ 4s¹ (29 electrons)• Remove 1 electron from 4s (highest n): [Ar] 3d¹⁰ (28 electrons) - 6
Calculation
(a) Cr unpaired electrons:• 3d⁵: 5 orbitals, each singly occupied → 5 unpaired• 4s¹: 1 orbital, singly occupied → 1 unpaired• Total: 6 unpaired electrons
(b) Cu⁺ unpaired electrons:• 3d¹⁰: 5 orbitals, each doubly occupied → 0 unpaired• Total: 0 unpaired electrons - 7
Final answer
| Species | Configuration | Unpaired e⁻ |
|---------|--------------|-------------|
| Cr | [Ar] 3d⁵ 4s¹ | 6 |
| Cu⁺ | [Ar] 3d¹⁰ | 0 |
Note: Z values (24, 29) are counting integers (exact) and do not affect any significant-figure consideration in this problem. - 8
Common trap
Writing Cr as [Ar] 3d⁴ 4s² (forgetting the anomaly) loses the mark AND earns a negative marking penalty. For Cu⁺, the trap is removing the 3d electron instead of 4s — always remove from highest n first in ions.
- 9
Similar NEET-style question
"The number of unpaired electrons in Cu²⁺ (Z = 29) is ___." [Answer: 1. Configuration: [Ar] 3d⁹. Nine d-electrons fill four orbitals doubly and one singly.]
What to remember before solving Electronic Configuration questions
Where do students lose marks on Electronic Configuration?
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student writes Cr as [Ar]3d⁴4s² (expected) instead of actual [Ar]3d⁵4s¹. Same for Cu: actual [Ar]3d¹⁰4s¹ (one e⁻ promoted from 4s to 3d).
When it triggers
Question asks for ground-state electronic configuration of Cr (Z=24) or Cu (Z=29).
How to avoid
Half-filled (d⁵) and fully filled (d¹⁰) configurations have extra stability from exchange energy and symmetry. Cr and Cu adopt these configurations by promoting one 4s electron.
More in Structure of Atom: 3 exam traps and mistakes · 5 formulas · 2 question patterns from its other lessons.
Electronic Configuration questions from past NEET papers
1 question from NEET 2021. Answers verified against NTA official keys.
NEET 2021
From the following pairs of ions which one is not an iso-electronic pair?
1Fe²⁺, Mn²⁺
2O²⁻, F⁻
3Na⁺, Mg²⁺
4Mn²⁺, Fe³⁺
NTA Answer: Option 1(final)
How does NEET ask about Electronic Configuration?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Write electronic configuration, especially handling Cr/Cu anomalies and ions.
InterpretationMedium
Common distractors
misses cr cu anomaly
Writes 3d⁴4s² instead of 3d⁵4s¹
Sources
NCERT refs: Class 11 Chemistry Chapter 2, p.65
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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