Aufbau Pauli Hund

8 MCQs9-step worked example

Source: NCERT Structure of AtomPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The trap that costs marks here: You apply the Aufbau principle mechanically — fill 1s, 2s, 2p, 3s, 3p, 4s, 3d in sequence — and write Cr as [Ar] 3d⁴ 4s². The actual ground-state configuration is [Ar] 3d⁵ 4s¹. NEET exploits this directly.

The three rules governing electron filling:

Aufbau principle — Electrons occupy the lowest available energy orbital first. The filling order follows (n + l) rule: lower (n + l) fills first; for equal (n + l), lower n fills first. This gives the sequence: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → ...

Pauli exclusion principle — No two electrons in an atom can have the same set of four quantum numbers (n, l, mₗ, mₛ). Consequence: each orbital holds a maximum of 2 electrons with opposite spins.

Hund's rule of maximum multiplicity — Electrons occupy degenerate orbitals (same subshell) singly with parallel spins before pairing begins. In the 2p subshell (three orbitals), the fourth electron pairs — the first three go one each.

The Cr/Cu anomaly (NCERT Class 11 Chemistry Chapter 2, page 36):

Half-filled (d⁵) and fully filled (d¹⁰) d-subshells have extra stability due to exchange energy and symmetrical charge distribution. Chromium (Z = 24) adopts [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s². Copper (Z = 29) adopts [Ar] 3d¹⁰ 4s¹ instead of [Ar] 3d⁹ 4s².

Watch-out: When writing configurations of ions (Cr³⁺, Cu²⁺), electrons are removed from 4s first (higher n), then 3d. Cr³⁺ = [Ar] 3d³. Cu²⁺ = [Ar] 3d⁹.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which of the following represents the correct ground-state electronic configuration of chromium (Z = 24)?

MCQ 2Direct ApplicationPractice

The ground-state electronic configuration of Cu (Z = 29) is [Ar] 3d¹⁰ 4s¹. What is the configuration of Cu²⁺?

MCQ 3Easy RecallPractice

According to Hund's rule, the electronic configuration of nitrogen (Z = 7) in the ground state has the 2p electrons arranged as:

MCQ 4Direct ApplicationPractice

Which of the following sets of quantum numbers is NOT possible for an electron?

MCQ 5Direct ApplicationPractice

The Aufbau principle predicts the filling order based on the (n + l) rule. Which of the following orbitals is filled BEFORE 4d?

MCQ 6Concept TrapPractice

An atom has the electronic configuration [Ar] 3d⁵ 4s¹. This configuration is adopted because:

MCQ 7CalculationPractice

The maximum number of electrons that can have the quantum numbers n = 3 and mₛ = +½ is:

MCQ 8Easy RecallPractice

Which of the following electronic configurations violates the Pauli exclusion principle?

Worked Example

Pattern: P.CHE.U02.AUFBAU_ELECTRONIC_CONFIG — Write electronic configuration handling the Cr/Cu anomaly.

1
Given
Element: Chromium, Z = 24.
2
Required
Ground-state electronic configuration of Cr.
3
Concept
Apply the Aufbau principle with (n + l) filling order. Recognize that half-filled (d⁵) configurations have extra stability — Cr is a known anomaly where one 4s electron promotes to 3d.
4
Formula/Rule
Aufbau filling order: 1s → 2s → 2p → 3s → 3p → 4s → 3d. Anomaly rule: half-filled d⁵ → extra exchange-energy stability → one 4s electron promotes.
5
Substitution / Setup
Expected (mechanical Aufbau): [Ar] 3d⁴ 4s² (24 − 18 = 6 electrons after Ar; 2 in 4s, 4 in 3d). Apply anomaly: promote one 4s electron to 3d → [Ar] 3d⁵ 4s¹.
6
Calculation / Verification
Electron count check: 18 (Ar) + 5 (3d) + 1 (4s) = 24 ✓ d⁵ = half-filled (5 electrons in 5 orbitals, one per orbital with parallel spins per Hund's rule) ✓
7
Final answer
Cr (Z = 24): **[Ar] 3d⁵ 4s¹**
8
Common trap
Writing [Ar] 3d⁴ 4s² by mechanically applying Aufbau without recognizing the half-filled stability exception. NEET 2024 tested this directly.
9
Similar NEET-style question
"The ground-state electronic configuration of Cu⁺ (Z = 29) is:" Apply: Cu = [Ar] 3d¹⁰ 4s¹ (fully filled anomaly). Remove the 4s electron for Cu⁺ → [Ar] 3d¹⁰. ---

Before solving, remember these

Law

Aufbau, Pauli, Hund's rules

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. 36

Formulas

Bohr energy (hydrogen-like)

E_{n} = - 13.6 \frac{Z ^{2}}{n ^{2}} eV

Energy of nth orbit. Negative (bound). Ground state H: -13.6 eV.

Symbol	Quantity	SI Unit
E_n	orbit energy	eV
Z	nuclear charge	-
n	principal	-

Valid when

Hydrogen-like atom
Non-relativistic

Bohr radius (hydrogen-like)

r_{n} = (0.529 \overset{˚}{A}) \frac{n ^{2}}{Z}

Radius of nth Bohr orbit for hydrogen-like atom of nuclear charge Z.

Symbol	Quantity	SI Unit
n	principal quantum number	-
Z	nuclear charge	-
r_n	orbit radius	Å

Valid when

Hydrogen-like (one-electron) atom
Non-relativistic

de Broglie wavelength

λ = h / (m v)

Wavelength associated with moving particle of momentum mv.

Symbol	Quantity	SI Unit
h	Planck 6.626e-34	J*s
m	mass	kg
v	velocity	m/s

Valid when

Non-relativistic

Heisenberg uncertainty

Δ x Δ p \geq \frac{h}{4 π}

Position and momentum cannot both be known with arbitrary precision.

Symbol	Quantity	SI Unit
Δx	position uncertainty	m
Δp	momentum uncertainty	kg*m/s

Valid when

Quantum scale; meaningful only when Δx, Δp comparable to atomic dimensions

Rydberg formula (H spectrum)

\frac{1}{λ} = R_{H} Z^{2} (1/ n_{1}^{2} - 1/ n_{2}^{2})

Spectral wavelengths of hydrogen-like atoms. Lyman (n1=1, UV), Balmer (n1=2, visible), Paschen (n1=3, IR).

Symbol	Quantity	SI Unit
lambda	wavelength	m
R_H	Rydberg 1.097e7	1/m
Z	nuclear charge	-
n1, n2	integers, n2>n1	-

Valid when

One-electron atom

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Cr / Cu electronic configuration anomaly

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.

Trap

Hydrogen-like Z² factor

Category: Similar Terms

Student forgets Z² scaling when applying Bohr formulas to He⁺ (Z=2) or Li²⁺ (Z=3).

When it triggers

Question involves hydrogen-like ion (He+, Li2+, etc.).

How to avoid

E_n = -13.6 × Z²/n² eV. r_n = (0.529/Z) × n² Å. He+: 4× more bound than H. Li²⁺: 9× more bound. Always include Z².

Mistake

Writes Cr as [Ar]3d⁴4s² (expected) instead of actual [Ar]3d⁵4s¹. Same for Cu.

Root cause: concept gap

Correction

Half-filled (d⁵) and full-filled (d¹⁰) configurations have extra exchange-energy stability. Cr and Cu adopt these by promoting one 4s electron.

Mistake

Drops Z² factor when applying Bohr formulas to He⁺ or Li²⁺.

Root cause: formula misuse

Correction

Always include Z². E_n = -13.6 × Z²/n². For He+: 4× more energetic than H. For Li²⁺: 9×.

Past Year Questions

9 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Higher yield of NO in - 40 - N 2 (g ) + O 2 (g ) 2 N O (g ) can be obtained at [H of the reaction = +180.7 kJ mol–1] A. Higher temperature B. Lower temperature C. Higher concentration of N 2 D. Higher concentration of O 2 Choose the correct answer from the options given below :

1A, C, D only

2A, D only

3B, C only

4B, C, D only

NTA Answer: Option 1(final)

NEET 2024Revised key

Match List I with List II. List I List II (Molecule) (Number and types of bond/s between two carbon atoms) A. ethane I. one -bond and two -bonds B. ethene II. two -bonds C. carbon molecule, C III. one -bonds 2 D. ethyne IV. one -bond and one -bond Choose the correct answer from the options given below:

1A-I, B-IV, C-II, D-III

2A-IV, B-III, C-II, D-I

3A-III, B-IV, C-II, D-I

4A-III, B-IV, C-I, D-II

NTA Answer: Option 3(revised_final)

NEET 2024Revised key

Match List I with List II List I List II (Quantum Number) (Information provided) A. m I. Shape of orbital l B. m II. Size of orbital s C. l III. Orientation of orbital D. n IV. Orientation of spin of electron Choose the correct answer from the options given below :

1A-I, B-III, C-II, D-IV

2A-III, B-IV, C-I, D-II

3A-III, B-IV, C-II, D-I

4A-II, B-I, C-IV, D-III

NTA Answer: Option 2(revised_final)

NEET 2024Revised key

Given below are two statements : Statement I : [Co(NH ) ]3+ is a homoleptic complex whereas [Co(NH ) Cl ]+ is a heteroleptic complex. 3 6 3 4 2 Statement II : Complex [Co(NH ) ]3+ has only one kind of ligands but [Co(NH ) Cl ]+ has more than one kind 3 6 3 4 2 of ligands. In the light of the above statements, choose the correct answer from the options given below.

1Both Statement I and Statement II are true

2Both Statement I and Statement II are false

3Statement I is true but Statement II is false

4Statement I is false but Statement II is true

NTA Answer: Option 1(revised_final)

NEET 2023

Select the correct statements from the following A. Atoms of all elements are composed of two fundamental particles. B. The mass of the electron is 9.10939 × 10–31 kg. C. All the isotopes of a given element show same chemical properties: D. Protons and electrons are collectively known as nucleons. E. Dalton’s atomic theory, regarded the atom as an ultimate particles of matter Choose the correct answer from the options given below

1C, D and E only

2A and E only

3B, C and E only

4A, B and C only

NTA Answer: Option 3(final)

NEET 2023

On balancing the given redox reaction, c aCrO2 bSO2(aq)cH(aq) 2aCr3(aq)bSO2(aq) H O(l) 2 7 3 4 2 2 the coefficients a, b and c are found to be, respectively-

13, 8, 1

21, 8, 3

38, 1, 3

41, 3, 8

NTA Answer: Option 4(final)

NEET 2022

Match List-I with List-II List-I List-II (a) Li (i) absorbent for carbon dioxide (b) Na (ii) electrochemical cells (c) KOH (iii) coolant in fast breeder reactors (d) Cs (iv) photoelectric cell Choose the correct answer from the options given below :

1(a) - (ii), (b) – (iii), (c) – (i), (d) – (iv)

2(a) - (iv), (b) – (i), (c) – (iii), (d) – (ii)

3(a) - (iii), (b) – (iv), (c) – (ii), (d) – (i)

4(a) - (i), (b) – (iii), (c) – (iv), (d) – (ii)

NTA Answer: Option 1(final)

NEET 2021

Statement I : Acid strength increases in the order given as HF << HCl << HBr << HI. Statement II : As the size of the elements F, Cl, Br, I increases down the group, the bond strength of HF, HCl, HBr and HI decreases and so the acid strength increases. In the light of the above statements, choose the correct answer from the options given below.

1Statement I is incorrect but Statement II is true

2Both statement I and Statement II are true

3Both Statement I and Statement II are false

4Statement I : correct but statement II is false

NTA Answer: Option 2(final)

NEET 2021

From the following pairs of ions which one is not an iso-electronic pair? Fe2+, Mn2+

1O2–, F–

2Na+, Mg2+

3Mn2+, Fe3+

4Answer

NTA Answer: Option 1(final)

How NEET usually asks this

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

Bohr energy difference between two levels; compute photon energy/wavelength using Rydberg or E_n formulas.

Multi StepMedium

Common distractors

forgets z squared

Drops Z² for hydrogen-like

Apply Δx·Δp ≥ h/(4π) to find minimum uncertainty given the other.

Direct ApplicationEasy

Common distractors

uses h instead of h over 4pi

Drops 4π factor

Given orbital (e.g. 3p_z) or electron, identify n, l, m_l, m_s. Or check forbidden quantum number combinations.

InterpretationEasy

Common distractors

uses wrong l range

Uses l ≤ n instead of l < n

Sources

NCERT refs: Class 11 Chemistry Chapter 2, p.36

Test yourself on this topic with real past-paper questions:

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