Filling of Orbitals: Aufbau Principle, Pauli Exclusion Principle & Hund's Rule | Class 11 Chemistry NEET Notes
Filling of Orbitals Class 11 Notes | Aufbau, Pauli & Hund's Rule
Filling of Orbitals in an Atom (Aufbau Principle) – Easy NEET Notes
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| Easy infographic explaining Aufbau Principle, Pauli Exclusion Principle and Hund's Rule for Class 11 Chemistry and NEET preparation. |
Filling of Orbitals
- Electrons enter orbitals in a fixed order.
- This process is called filling of orbitals.
- The order depends on the energy of orbitals.
- Lower energy orbitals are filled first, then higher energy orbitals.
- The filling follows three important rules:
- Aufbau Principle
- Pauli Exclusion Principle
- Hund's Rule of Maximum Multiplicity
Aufbau Principle
Meaning of Aufbau
- The German word "Aufbau" means "building up".
- It means electrons build the electronic configuration step by step.
Definition
- Electrons first occupy the lowest energy orbital available.
- Only after lower energy orbitals are completely filled do electrons move to higher energy orbitals.
Easy Example
- 1s fills before 2s
- 2s fills before 2p
- 2p fills before 3s
So, electrons always choose the lowest energy orbital first.
Why do electrons fill lower energy orbitals first?
- Every electron tries to remain in the lowest possible energy state.
- Lower energy means greater stability.
- Therefore, atoms naturally fill lower energy orbitals first.
Energy of Orbitals
- The energy of an orbital depends on:
- Effective nuclear charge (Zeff)
- Distance from the nucleus
- Type of orbital (s, p, d, f)
Important Point (NEET):
- The energy order is not always based on principal quantum number (n).
- Example:
- 4s has lower energy than 3d, so 4s fills first.
Order of Filling of Orbitals (Very Important)
Memorize this order:
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s
Easy Memory Sequence
1s
↓
2s
↓
2p
↓
3s
↓
3p
↓
4s
↓
3d
↓
4p
↓
5s
↓
4d
↓
5p
↓
6s
↓
4f
↓
5d
↓
6p
↓
7s
Example: Potassium (K)
- Atomic number = 19
- First 18 electrons fill up to 3p.
- The 19th electron has two choices:
- 3d
- 4s
- Since 4s has lower energy, the electron enters 4s.
Electronic configuration:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Important NEET Fact:
4s fills before 3d.
Important Points
- Aufbau principle gives the order of filling, not the actual energy after filling.
- It is correct for most elements.
- Some atoms show exceptions because the energies of some orbitals are very close.
Examples of exceptions:
- Chromium (Cr)
- Copper (Cu)
These are important NEET exceptions.
Why do exceptions occur?
- Sometimes two orbitals have almost equal energy.
- A small change in atomic structure can change the filling order.
- Half-filled and completely filled d-orbitals are more stable.
NEET Important Facts
- Lowest energy orbital = 1s
- 4s fills before 3d
- Electrons always fill lower energy orbitals first
- Aufbau = Building up
- Exceptions: Cr and Cu
- Energy order ≠ Principal quantum number order
Quick Revision
- Aufbau means Building Up.
- Electrons fill lowest energy orbitals first.
- Filling order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s
- 4s is filled before 3d.
- Aufbau principle follows:
- Pauli Exclusion Principle
- Hund's Rule
- Some exceptions occur because orbitals have nearly equal energies (e.g., Cr and Cu).
One-Line NEET Definition
Aufbau Principle: In the ground state of an atom, electrons occupy orbitals in the order of increasing energy, so lower-energy orbitals are filled before higher-energy orbitals.
Pauli Exclusion Principle – Easy NEET Notes (Line-by-Line Explanation)
Pauli Exclusion Principle
Introduction
- The number of electrons that can enter an orbital is limited.
- This limit is explained by the Pauli Exclusion Principle.
- This principle was given by Austrian scientist Wolfgang Pauli in 1926.
- It is one of the three important rules for writing electronic configuration.
Definition of Pauli Exclusion Principle
Definition (NEET):
"No two electrons in an atom can have the same set of four quantum numbers."
Easy Meaning
- Every electron in an atom has its own unique identity.
- No two electrons can have all four quantum numbers (n, l, ml, ms) exactly the same.
Simple Statement (Most Important)
Pauli Exclusion Principle can also be written as:
"Only two electrons can occupy the same orbital, and they must have opposite spins."
This is the most commonly asked statement in NEET.
What does "opposite spin" mean?
Every electron has one of two possible spins:
- Spin Up (↑) = +½
- Spin Down (↓) = –½
So, one orbital can contain:
↑↓
But it cannot contain:
↑↑ ❌
or
↓↓ ❌
Understanding the Four Quantum Numbers
Every electron is described by four quantum numbers:
- Principal Quantum Number (n) → Shell
- Azimuthal Quantum Number (l) → Subshell
- Magnetic Quantum Number (ml) → Orbital
- Spin Quantum Number (ms) → Spin direction
Which Quantum Number Must Be Different?
Two electrons in the same orbital have:
- Same n ✔
- Same l ✔
- Same ml ✔
But they must have:
- Different ms (spin quantum number) ✔
One electron:
ms = +½
Second electron:
ms = –½
This is why they can stay together in one orbital.
Maximum Number of Electrons in One Orbital
One orbital contains only two electrons.
Condition:
- One electron = ↑
- Second electron = ↓
So,
Maximum electrons in one orbital = 2
Capacity of Different Subshells
s Subshell
- Number of orbitals = 1
- Each orbital holds 2 electrons
Maximum electrons:
1 × 2 = 2
s subshell = 2 electrons
p Subshell
- Number of orbitals = 3
- Each orbital holds 2 electrons
Maximum electrons:
3 × 2 = 6
p subshell = 6 electrons
d Subshell
- Number of orbitals = 5
- Each orbital holds 2 electrons
Maximum electrons:
5 × 2 = 10
d subshell = 10 electrons
f Subshell
- Number of orbitals = 7
- Each orbital holds 2 electrons
Maximum electrons:
7 × 2 = 14
f subshell = 14 electrons
Capacity of Different Subshells (Table)
| Subshell | Number of Orbitals | Maximum Electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Maximum Number of Electrons in a Shell
The formula is:
Maximum electrons = 2n²
Where n = Principal Quantum Number
Examples:
- n = 1 → 2 × (1²) = 2
- n = 2 → 2 × (2²) = 8
- n = 3 → 2 × (3²) = 18
- n = 4 → 2 × (4²) = 32
Example
1s Orbital
There is only one orbital.
It can hold:
↑↓
Maximum = 2 electrons
Not possible:
↑↑ ❌
because both electrons would have the same spin.
Why is Pauli Exclusion Principle Important?
- It limits the number of electrons in each orbital.
- It helps determine the electronic configuration of atoms.
- It explains why atoms have different electron arrangements.
- It is used to calculate the maximum capacity of shells and subshells.
Important NEET Facts
- No two electrons in an atom have the same four quantum numbers.
- One orbital can hold only two electrons.
- The two electrons must have opposite spins.
- Same orbital → Same n, l, ml, but different ms.
- Maximum electrons in one orbital = 2.
- Maximum electrons in a shell = 2n².
- s = 2, p = 6, d = 10, f = 14.
Quick Revision
- Scientist: Wolfgang Pauli (1926)
- Rule: No two electrons have the same four quantum numbers.
- One orbital = Maximum 2 electrons
- Spins must be opposite (↑↓)
- Formula: Maximum electrons in shell = 2n²
- Subshell capacities: s = 2, p = 6, d = 10, f = 14
One-Line NEET Definition
Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers; therefore, one orbital can contain a maximum of two electrons with opposite spins (↑↓).
Hund's Rule of Maximum Multiplicity – Easy NEET Notes (Line-by-Line Explanation)
Hund's Rule of Maximum Multiplicity
Introduction
- Hund's Rule explains how electrons fill orbitals of the same energy.
- It is used only for degenerate orbitals.
- This rule is one of the three important rules for writing electronic configuration.
- It helps us understand the correct arrangement of electrons in p, d, and f subshells.
What are Degenerate Orbitals?
- Degenerate orbitals are orbitals that have the same energy.
- Electrons fill these orbitals according to Hund's Rule.
Examples
- p subshell → 3 orbitals (px, py, pz)
- d subshell → 5 orbitals
- f subshell → 7 orbitals
All orbitals within the same subshell have equal energy.
Definition of Hund's Rule
Definition (NEET):
"Pairing of electrons in the orbitals of the same subshell does not occur until each orbital has one electron."
Easy Meaning
- Electrons first occupy empty orbitals one by one.
- Only after every orbital has one electron does pairing begin.
- This arrangement gives maximum stability to the atom.
Easy Trick
Single First → Pair Later
or
One seat for each student before anyone shares a seat.
This is the easiest way to remember Hund's Rule.
Filling of p Orbitals
A p subshell has 3 orbitals.
First Electron
↑ _ _
Second Electron
↑ ↑ _
Third Electron
↑ ↑ ↑
Now every orbital has one electron.
Fourth Electron
↑↓ ↑ ↑
Pairing starts with the 4th electron.
Fifth Electron
↑↓ ↑↓ ↑
Sixth Electron
↑↓ ↑↓ ↑↓
Now the p subshell is completely filled.
Important Point
In the p subshell:
- Pairing starts with the 4th electron.
Filling of d Orbitals
A d subshell has 5 orbitals.
Electrons fill like this:
↑ ↑ ↑ ↑ ↑
First, all 5 orbitals receive one electron each.
Then pairing begins.
So,
- Pairing starts with the 6th electron.
Filling of f Orbitals
An f subshell has 7 orbitals.
Electrons fill one by one:
↑ ↑ ↑ ↑ ↑ ↑ ↑
After all 7 orbitals contain one electron, pairing begins.
So,
- Pairing starts with the 8th electron.
Pairing Starts At
| Subshell | Number of Orbitals | Pairing Starts With |
|---|---|---|
| p | 3 | 4th electron |
| d | 5 | 6th electron |
| f | 7 | 8th electron |
This table is very important for NEET.
Why do electrons remain unpaired first?
- Electrons repel each other because they have the same negative charge.
- By occupying separate orbitals first, they stay farther apart.
- This reduces electron-electron repulsion.
- Therefore, the atom becomes more stable.
Half-Filled Orbitals
When every orbital of a subshell contains one electron, it is called a half-filled subshell.
Examples:
- p³
- d⁵
- f⁷
Half-filled subshells are more stable.
Fully Filled Orbitals
When every orbital contains two electrons, it is called a fully filled subshell.
Examples:
- p⁶
- d¹⁰
- f¹⁴
Fully filled subshells are also more stable.
Why are Half-Filled and Fully Filled Subshells More Stable?
They have:
- Better symmetry.
- Lower electron-electron repulsion.
- Greater exchange energy (an important concept that increases stability).
Therefore,
Half-filled and fully filled subshells have extra stability.
Example
Nitrogen (Z = 7)
Electronic configuration:
1s² 2s² 2p³
The three p electrons fill as:
↑ ↑ ↑
Not
↑↓ ↑ _ ❌
because Hund's Rule says single occupancy first.
Example
Oxygen (Z = 8)
Electronic configuration:
1s² 2s² 2p⁴
Arrangement:
↑↓ ↑ ↑
Pairing begins only after each orbital has one electron.
Why is Hund's Rule Important?
- It gives the correct electronic configuration.
- It explains magnetic properties of atoms.
- It helps explain the extra stability of half-filled and fully filled subshells.
- It is essential for solving NEET electron configuration questions.
Important NEET Facts
- Hund's Rule applies only to degenerate (equal-energy) orbitals.
- Electrons occupy orbitals singly first.
- Pairing starts only after each orbital has one electron.
- p → Pairing starts with 4th electron
- d → Pairing starts with 6th electron
- f → Pairing starts with 8th electron
- Half-filled and fully filled subshells are extra stable.
Quick Revision
- Hund's Rule: Single occupancy first, pairing later.
- Applies to p, d, and f subshells.
- Degenerate orbitals = Orbitals with equal energy.
- Pairing starts:
- p → 4th electron
- d → 6th electron
- f → 8th electron
- Half-filled (p³, d⁵, f⁷) and fully filled (p⁶, d¹⁰, f¹⁴) subshells are more stable.
One-Line NEET Definition
Hund's Rule of Maximum Multiplicity: In a set of degenerate orbitals, electrons occupy each orbital singly with parallel spins before any pairing occurs, giving maximum stability to the atom.
Filling of Orbitals
│
├── Aufbau Principle
│ │
│ ├── Meaning
│ │ ├── Aufbau = "Building Up"
│ │ └── Electrons fill orbitals step by step
│ │
│ ├── Main Rule
│ │ ├── Lowest energy orbital fills first
│ │ └── Higher energy orbital fills later
│ │
│ ├── Reason
│ │ ├── Lower energy = More stable
│ │ └── Atom prefers minimum energy
│ │
│ ├── Filling Order
│ │ ├── 1s
│ │ ├── 2s
│ │ ├── 2p
│ │ ├── 3s
│ │ ├── 3p
│ │ ├── 4s
│ │ ├── 3d
│ │ ├── 4p
│ │ ├── 5s
│ │ ├── 4d
│ │ ├── 5p
│ │ ├── 6s
│ │ ├── 4f
│ │ ├── 5d
│ │ ├── 6p
│ │ └── 7s
│ │
│ ├── Important Example
│ │ ├── Potassium (Z = 19)
│ │ ├── 4s fills before 3d
│ │ └── Configuration = 4s¹
│ │
│ └── Exceptions
│ ├── Cr
│ ├── Cu
│ └── Due to nearly equal energies
Pauli Exclusion Principle
│
├── Scientist
│ ├── Wolfgang Pauli
│ └── 1926
│
├── Main Rule
│ ├── No two electrons have the same
│ │ four quantum numbers
│ └── One orbital holds only 2 electrons
│
├── Spin Rule
│ ├── Opposite spins only
│ ├── ↑↓ = Allowed
│ ├── ↑↑ = Not Allowed
│ └── ↓↓ = Not Allowed
│
├── Quantum Numbers
│ ├── n = Same
│ ├── l = Same
│ ├── ml = Same
│ └── ms = Must be Different
│
├── Orbital Capacity
│ ├── One orbital = 2 electrons
│ ├── s = 2
│ ├── p = 6
│ ├── d = 10
│ └── f = 14
│
├── Shell Capacity
│ ├── Formula = 2n²
│ ├── n=1 → 2
│ ├── n=2 → 8
│ ├── n=3 → 18
│ └── n=4 → 32
│
└── Importance
├── Limits electrons
├── Electronic configuration
└── Maximum capacity of shells
Hund's Rule
│
├── Applies To
│ ├── Degenerate orbitals
│ ├── p
│ ├── d
│ └── f
│
├── Main Rule
│ ├── Single occupancy first
│ ├── Pairing later
│ └── Parallel spins first
│
├── Easy Trick
│ ├── Single First
│ └── Pair Later
│
├── Pairing Starts
│ ├── p → 4th electron
│ ├── d → 6th electron
│ └── f → 8th electron
│
├── Stability
│ ├── Half-filled = Extra stable
│ ├── Fully filled = Extra stable
│ ├── Better symmetry
│ └── Lower repulsion
│
├── Examples
│ ├── p³ = Half-filled
│ ├── p⁶ = Fully filled
│ ├── d⁵ = Half-filled
│ ├── d¹⁰ = Fully filled
│ ├── f⁷ = Half-filled
│ └── f¹⁴ = Fully filled
│
└── NEET Points
├── Equal-energy orbitals only
├── One electron in each orbital first
├── Pairing after all are singly occupied
└── Gives maximum stability
CBSE Class 11 Chemistry Question Bank
Part 1: Aufbau Principle
Section A: Multiple Choice Questions (MCQs)
1. The Aufbau principle states that electrons fill orbitals according to
A. Increasing energy
B. Decreasing energy
C. Random order
D. Alphabetical order
Answer: A
2. The word "Aufbau" means
A. Pairing B. Building up C. Spinning D. Filling equally
Answer: B
3. Which orbital is filled first?
A. 2s B. 2p C. 1s D. 3s
Answer: C
4. Which orbital is filled before 3d?
A. 4p B. 4s C. 5s D. 3p
Answer: B
5. Which is the correct filling order?
A. 1s → 2s → 2p → 3s
B. 1s → 2p → 2s → 3s
C. 1s → 3s → 2s → 2p
D. 2s → 1s → 2p → 3s
Answer: A
6. Potassium has its nineteenth electron in
A. 3d
B. 4s
C. 4p
D. 5s
Answer: B
7. Which orbital has the lowest energy?
A. 2s
B. 2p
C. 3s
D. 1s
Answer: D
8. Aufbau principle is mainly based on
A. Increasing energy
B. Increasing spin
C. Increasing mass
D. Increasing charge
Answer: A
9. Which of the following is filled after 5p?
A. 5d
B. 6s
C. 6p
D. 4f
Answer: B
10. Which orbital is filled after 6s?
A. 6p
B. 5d
C. 4f
D. 7s
Answer: C
Answers
1-A 2-B 3-C 4-B 5-A 6-B 7-D 8-A 9-B 10-C
Section B: Fill in the Blanks
- Aufbau is a ______ word.
Answer: German
- Aufbau means ________.
Answer: Building up
- Electrons first occupy the ______ energy orbital.
Answer: Lowest
- The first orbital filled is ______.
Answer: 1s
- The orbital filled before 3d is ______.
Answer: 4s
- The filling order follows ______ energy.
Answer: Increasing
- Potassium has its last electron in ______ orbital.
Answer: 4s
- Aufbau principle is followed in the ______ state.
Answer: Ground
- The energy of an orbital depends upon ______ nuclear charge.
Answer: Effective
- Chromium and copper are ______ to Aufbau principle.
Answer: Exceptions
Section C: Very Short Answer Questions (1 Mark)
1. What is Aufbau principle?
Answer: Electrons fill orbitals in order of increasing energy.
2. What does Aufbau mean?
Answer: Building up.
3. Which orbital fills first?
Answer: 1s orbital.
4. Which orbital fills before 3d?
Answer: 4s orbital.
5. Why do electrons fill lower-energy orbitals first?
Answer: Because lower-energy orbitals are more stable.
6. Write the first four orbitals in filling order.
Answer: 1s → 2s → 2p → 3s
7. Which electron enters 4s in potassium?
Answer: 19th electron.
8. Is Aufbau principle applicable to ground state?
Answer: Yes.
9. Name one exception to Aufbau principle.
Answer: Chromium (Cr)
10. Name another exception.
Answer: Copper (Cu)
Section D: Short Answer Questions (2–3 Marks)
1. State Aufbau principle.
Answer:
Electrons occupy orbitals in the order of increasing energy.
Lower-energy orbitals fill before higher-energy orbitals.
2. Why is 4s filled before 3d?
Answer:
The 4s orbital has slightly lower energy than the 3d orbital before electron filling.
Therefore, electrons enter 4s first.
3. Write the order of filling of orbitals up to 5p.
Answer:
1s
2s
2p
3s
3p
4s
3d
4p
5s
4d
5p
4. Why do exceptions occur in Aufbau principle?
Answer:
Some orbitals have nearly equal energies.
Half-filled and fully filled subshells are more stable.
Hence exceptions occur.
5. Why is Aufbau principle useful?
Answer:
It helps predict electronic configurations.
It explains electron arrangement.
It is useful for studying periodic properties.
Section E: Long Answer Questions (5 Marks)
1. Explain Aufbau principle with suitable examples.
Answer:
• Aufbau means "building up."
• Electrons fill orbitals in increasing order of energy.
• Lowest-energy orbitals are occupied first.
• Order:
1s
2s
2p
3s
3p
4s
3d
4p
5s
4d
5p
6s
4f
5d
6p
7s
Example:
Potassium (Z = 19)
Electronic configuration:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
The nineteenth electron enters 4s because it has lower energy than 3d.
2. Explain the significance of Aufbau principle.
Answer:
• Determines electronic configuration.
• Helps understand periodic table.
• Explains chemical properties.
• Helps predict valency.
• Basis of modern atomic structure.
Section F: Assertion–Reason Questions
1.
Assertion: Electrons occupy lower-energy orbitals first.
Reason: Lower-energy orbitals are more stable.
Answer: Both A and R are true and R is the correct explanation.
2.
Assertion: 4s fills before 3d.
Reason: 4s has lower energy than 3d.
Answer: Both true.
3.
Assertion: Chromium follows Aufbau principle completely.
Reason: Chromium has half-filled d orbital.
Answer: Assertion is false; Reason is true.
4.
Assertion: Aufbau principle applies in ground state.
Reason: Electrons seek minimum energy.
Answer: Both true.
5.
Assertion: 1s fills after 2s.
Reason: 1s has higher energy.
Answer: Both false.
Section G: Statement-Based Questions
Choose the correct option.
1.
Statement I: Electrons occupy lowest-energy orbitals first.
Statement II: 4s fills before 3d.
A. Both true
B. Both false
C. Only I true
D. Only II true
Answer: A
2.
Statement I: Aufbau means building up.
Statement II: Electrons occupy higher-energy orbitals first.
Answer: Statement I true; Statement II false.
3.
Statement I: Potassium has one electron in 4s.
Statement II: 3d fills before 4s.
Answer: I true; II false.
Section H: Match the Columns
| Column A | Column B |
|---|---|
| Aufbau | Building up |
| Lowest orbital | 1s |
| Potassium | 4s¹ |
| Exception | Chromium |
| First d filling | After 4s |
Answers
1 → Building up
2 → 1s
3 → 4s¹
4 → Chromium
5 → After 4s
Section I: Case Study Questions
Case Study
A student is writing electronic configurations.
He knows electrons always occupy lower-energy orbitals first.
He writes:
1s²
2s²
2p⁶
3s²
3p⁶
4s¹
Answer the following.
Q1. Which principle is followed?
Answer: Aufbau Principle
Q2. Which element has this configuration?
Answer: Potassium
Q3. Why is 4s filled before 3d?
Answer: Because 4s has lower energy.
Q4. Is this a ground-state configuration?
Answer: Yes.
Q5. Name one exception to Aufbau principle.
Answer: Chromium (Cr) or Copper (Cu)
Important CBSE Questions
- Define Aufbau principle.
- Explain why 4s fills before 3d.
- Write the order of orbital filling.
- Explain exceptions to Aufbau principle.
- Why do electrons occupy lower-energy orbitals first?
- Write the electronic configuration of potassium.
- State the significance of Aufbau principle.
- Differentiate between Aufbau principle and Hund's rule.
- Why is Aufbau principle called the building-up principle?
- Write the filling order up to 7s.
CBSE Class 11 Chemistry Question Bank
Part 2: Pauli Exclusion Principle
Section A: Multiple Choice Questions (MCQs)
1. Who proposed the Pauli Exclusion Principle?
A. Bohr
B. Rutherford
C. Wolfgang Pauli
D. Schrödinger
Answer: C
2. The Pauli Exclusion Principle was proposed in
A. 1913
B. 1926
C. 1932
D. 1905
Answer: B
3. According to Pauli's principle, one orbital can contain a maximum of
A. 1 electron
B. 2 electrons
C. 4 electrons
D. 6 electrons
Answer: B
4. The two electrons in the same orbital must have
A. Same spin
B. Opposite spins
C. No spin
D. Any spin
Answer: B
5. No two electrons in an atom can have the same
A. Three quantum numbers
B. Four quantum numbers
C. Two quantum numbers
D. One quantum number
Answer: B
6. The maximum number of electrons in an s subshell is
A. 1
B. 2
C. 6
D. 10
Answer: B
7. The maximum number of electrons in a p subshell is
A. 2
B. 4
C. 6
D. 8
Answer: C
8. The maximum number of electrons in a d subshell is
A. 6
B. 8
C. 10
D. 14
Answer: C
9. The formula for the maximum number of electrons in the nth shell is
A. n²
B. 2n
C. 2n²
D. n³
Answer: C
10. Which spin values are allowed in one orbital?
A. ↑ ↑
B. ↓ ↓
C. ↑ ↓
D. None
Answer: C
Answer Key
1-C, 2-B, 3-B, 4-B, 5-B, 6-B, 7-C, 8-C, 9-C, 10-C
Section B: Fill in the Blanks
- Pauli Exclusion Principle was given by __________.
Answer: Wolfgang Pauli
- It was proposed in the year __________.
Answer: 1926
- One orbital can contain a maximum of __________ electrons.
Answer: Two
- The two electrons must have __________ spins.
Answer: Opposite
- No two electrons can have the same __________ quantum numbers.
Answer: Four
- The maximum number of electrons in an s subshell is __________.
Answer: 2
- The maximum number of electrons in a p subshell is __________.
Answer: 6
- The maximum number of electrons in a d subshell is __________.
Answer: 10
- The maximum number of electrons in an f subshell is __________.
Answer: 14
- Maximum electrons in the nth shell = __________.
Answer: 2n²
Section C: Very Short Answer Questions (1 Mark)
1. State the Pauli Exclusion Principle.
Answer: No two electrons in an atom can have the same set of four quantum numbers.
2. Who proposed this principle?
Answer: Wolfgang Pauli.
3. How many electrons can occupy one orbital?
Answer: Two electrons.
4. What should be the spin of two electrons in one orbital?
Answer: Opposite spins.
5. What is the maximum number of electrons in the p subshell?
Answer: 6 electrons.
6. What is the maximum number of electrons in the d subshell?
Answer: 10 electrons.
7. What is the maximum number of electrons in the f subshell?
Answer: 14 electrons.
8. Write the formula for the maximum number of electrons in a shell.
Answer: 2n²
9. What is the maximum number of electrons in the third shell?
Answer: 18 electrons.
10. What is the maximum number of electrons in the fourth shell?
Answer: 32 electrons.
Section D: Short Answer Questions (2–3 Marks)
1. State the Pauli Exclusion Principle.
Answer:
- No two electrons in an atom can have the same four quantum numbers.
- One orbital can contain only two electrons.
- The two electrons must have opposite spins.
2. Why can only two electrons occupy one orbital?
Answer: Because electrons in the same orbital have the same values of n, l and ml. Therefore, they must differ only in spin quantum number (ms). Hence, only two electrons with opposite spins can occupy one orbital.
3. Write the maximum number of electrons in different subshells.
Answer:
- s = 2
- p = 6
- d = 10
- f = 14
4. Write the maximum number of electrons in the first four shells.
Answer:
- n = 1 → 2
- n = 2 → 8
- n = 3 → 18
- n = 4 → 32
5. Why is the Pauli Exclusion Principle important?
Answer:
- Explains electron arrangement.
- Limits electrons in an orbital.
- Helps write electronic configurations.
- Explains shell and subshell capacities.
Section E: Long Answer Questions (5 Marks)
1. Explain the Pauli Exclusion Principle.
Answer:
- Proposed by Wolfgang Pauli in 1926.
- No two electrons in an atom can have the same set of four quantum numbers.
- One orbital can contain only two electrons.
- These electrons must have opposite spins (↑↓).
- Same orbital:
- n = same
- l = same
- ml = same
- ms = opposite
- This principle determines the maximum number of electrons in orbitals, subshells, and shells.
2. Explain the electron capacity of orbitals, subshells and shells.
Answer:
One orbital
- Maximum = 2 electrons
Subshells
- s = 2
- p = 6
- d = 10
- f = 14
Shells
- Formula = 2n²
- K shell = 2
- L shell = 8
- M shell = 18
- N shell = 32
Section F: Assertion–Reason Questions
1.
Assertion (A): Only two electrons can occupy one orbital.
Reason (R): The two electrons must have opposite spins.
Answer: Both A and R are true, and R is the correct explanation.
2.
Assertion (A): No two electrons in an atom have the same four quantum numbers.
Reason (R): This is the Pauli Exclusion Principle.
Answer: Both A and R are true, and R is the correct explanation.
3.
Assertion (A): An s subshell can contain six electrons.
Reason (R): It contains one orbital.
Answer: Assertion is false; Reason is true.
4.
Assertion (A): A p subshell contains three orbitals.
Reason (R): Each orbital can hold two electrons.
Answer: Both A and R are true, but R is not the correct explanation of A.
5.
Assertion (A): The maximum number of electrons in the third shell is 18.
Reason (R): Maximum electrons = 2n².
Answer: Both A and R are true, and R is the correct explanation.
Section G: Statement-Based Questions
1.
Statement I: One orbital can contain two electrons.
Statement II: The two electrons have opposite spins.
A. Both statements are true.
B. Both statements are false.
C. Statement I is true, II is false.
D. Statement I is false, II is true.
Answer: A
2.
Statement I: Maximum electrons in p subshell = 6.
Statement II: p subshell has three orbitals.
Answer: Both statements are true.
3.
Statement I: Maximum electrons in n = 3 shell = 18.
Statement II: Formula = 2n².
Answer: Both statements are true.
4.
Statement I: Same orbital electrons have opposite spins.
Statement II: Same orbital electrons have different values of n.
Answer: Statement I is true; Statement II is false.
5.
Statement I: No two electrons have identical four quantum numbers.
Statement II: Pauli Exclusion Principle explains this rule.
Answer: Both statements are true.
Section H: Match the Columns
| Column A | Column B |
|---|---|
| Wolfgang Pauli | 1926 |
| One orbital | 2 electrons |
| p subshell | 6 electrons |
| d subshell | 10 electrons |
| Shell capacity | 2n² |
Answers
1 → 1926
2 → 2 electrons
3 → 6 electrons
4 → 10 electrons
5 → 2n²
Section I: Case Study Questions
Case Study
A student writes the electronic configuration of an atom. He places two electrons in the same orbital as ↑↓. His teacher says this arrangement follows the Pauli Exclusion Principle.
Answer the following questions.
Q1. Which principle is followed?
Answer: Pauli Exclusion Principle.
Q2. How many electrons can occupy one orbital?
Answer: Two.
Q3. What should be the spin of the two electrons?
Answer: Opposite spins.
Q4. Who proposed this principle?
Answer: Wolfgang Pauli.
Q5. Write the formula for the maximum number of electrons in a shell.
Answer: 2n².
Important CBSE Questions
- State the Pauli Exclusion Principle.
- Who proposed the Pauli Exclusion Principle?
- Why can only two electrons occupy one orbital?
- Explain the significance of the Pauli Exclusion Principle.
- Write the maximum number of electrons in s, p, d and f subshells.
- Derive the formula 2n² for the maximum number of electrons in a shell.
- Differentiate between an orbital and a subshell based on electron capacity.
- Explain why electrons in the same orbital have opposite spins.
- Write the electronic capacities of the first four shells.
- Why is the Pauli Exclusion Principle important in writing electronic configurations?
CBSE Class 11 Chemistry Question Bank
Part 3: Hund's Rule of Maximum Multiplicity
Section A: Multiple Choice Questions (MCQs)
1. Hund's rule applies to
A. s orbitals only
B. Degenerate orbitals
C. All shells only
D. Nucleus
Answer: B
2. Degenerate orbitals are orbitals having
A. Different energies
B. Same energy
C. Different shapes only
D. Different spins
Answer: B
3. According to Hund's rule, electrons first occupy
A. One orbital with pairing
B. Different orbitals singly
C. Higher energy orbitals
D. Random orbitals
Answer: B
4. Pairing in p orbitals begins with the
A. 2nd electron
B. 3rd electron
C. 4th electron
D. 5th electron
Answer: C
5. Pairing in d orbitals begins with the
A. 4th electron
B. 5th electron
C. 6th electron
D. 7th electron
Answer: C
6. Pairing in f orbitals begins with the
A. 6th electron
B. 7th electron
C. 8th electron
D. 9th electron
Answer: C
7. A p subshell contains
A. 1 orbital
B. 2 orbitals
C. 3 orbitals
D. 5 orbitals
Answer: C
8. A d subshell contains
A. 3 orbitals
B. 4 orbitals
C. 5 orbitals
D. 7 orbitals
Answer: C
9. Half-filled subshells are
A. Less stable
B. More stable
C. Unstable
D. Neutral
Answer: B
10. Fully filled subshells are
A. Less stable
B. More stable
C. Unstable
D. Empty
Answer: B
Answer Key
1-B, 2-B, 3-B, 4-C, 5-C, 6-C, 7-C, 8-C, 9-B, 10-B
Section B: Fill in the Blanks
- Hund's rule applies to __________ orbitals.
Answer: Degenerate
- Degenerate orbitals have __________ energy.
Answer: Equal
- Electrons first occupy orbitals __________.
Answer: Singly
- Pairing begins only after __________ occupancy.
Answer: Single
- A p subshell contains __________ orbitals.
Answer: Three
- A d subshell contains __________ orbitals.
Answer: Five
- An f subshell contains __________ orbitals.
Answer: Seven
- Pairing in p starts with the __________ electron.
Answer: Fourth
- Half-filled orbitals are __________ stable.
Answer: More
- Fully filled orbitals have extra __________.
Answer: Stability
Section C: Very Short Answer Questions (1 Mark)
1. State Hund's Rule of Maximum Multiplicity.
Answer: Electrons occupy degenerate orbitals singly with parallel spins before pairing begins.
2. What are degenerate orbitals?
Answer: Orbitals having the same energy.
3. How many orbitals are present in a p subshell?
Answer: Three.
4. How many orbitals are present in a d subshell?
Answer: Five.
5. How many orbitals are present in an f subshell?
Answer: Seven.
6. When does pairing begin in p orbitals?
Answer: With the fourth electron.
7. When does pairing begin in d orbitals?
Answer: With the sixth electron.
8. When does pairing begin in f orbitals?
Answer: With the eighth electron.
9. Why are half-filled subshells stable?
Answer: Due to symmetrical electron distribution and lower electron repulsion.
10. Give one example of a half-filled subshell.
Answer: d⁵ or p³.
Section D: Short Answer Questions (2–3 Marks)
1. State Hund's Rule.
Answer:
- Electrons occupy degenerate orbitals singly first.
- Pairing starts only after every orbital has one electron.
- Electrons have parallel spins during single occupancy.
2. What are degenerate orbitals? Give examples.
Answer:
Degenerate orbitals are orbitals having the same energy.
Examples:
- Three p orbitals
- Five d orbitals
- Seven f orbitals
3. Explain the filling of electrons in p orbitals.
Answer:
Three p orbitals are filled one by one first.
Example:
↑ ↑ ↑
Only after this does pairing begin.
↑↓ ↑ ↑
4. Why are half-filled and fully filled orbitals more stable?
Answer:
- They have symmetrical distribution.
- Electron repulsion is minimum.
- They possess extra stability.
5. Write the order of filling of four electrons in p orbitals.
Answer:
1st → ↑ _ _
2nd → ↑ ↑ _
3rd → ↑ ↑ ↑
4th → ↑↓ ↑ ↑
Section E: Long Answer Questions (5 Marks)
1. Explain Hund's Rule of Maximum Multiplicity.
Answer:
Hund's Rule states that electrons occupy degenerate orbitals singly with parallel spins before pairing.
Important points:
- Applies to p, d and f orbitals.
- Electrons first occupy empty orbitals.
- Pairing begins only after single occupancy.
- This arrangement reduces electron repulsion.
- It increases stability.
Examples:
p orbitals
1 electron
↑ _ _
2 electrons
↑ ↑ _
3 electrons
↑ ↑ ↑
4 electrons
↑↓ ↑ ↑
2. Explain the importance of Hund's Rule.
Answer:
Hund's Rule:
- Gives correct electronic configuration.
- Explains magnetic properties.
- Explains stability of half-filled and fully filled orbitals.
- Reduces electron repulsion.
- Increases atomic stability.
Section F: Assertion–Reason Questions
1.
Assertion (A): Electrons occupy degenerate orbitals singly before pairing.
Reason (R): This arrangement reduces electron repulsion.
Answer: Both A and R are true, and R is the correct explanation.
2.
Assertion (A): Pairing starts with the fourth electron in p orbitals.
Reason (R): A p subshell contains three orbitals.
Answer: Both A and R are true, and R is the correct explanation.
3.
Assertion (A): Half-filled orbitals are more stable.
Reason (R): They possess symmetrical distribution.
Answer: Both A and R are true, and R is the correct explanation.
4.
Assertion (A): A d subshell contains five orbitals.
Reason (R): Pairing starts with the fifth electron.
Answer: Assertion is true, Reason is false.
5.
Assertion (A): Hund's Rule applies to degenerate orbitals.
Reason (R): Degenerate orbitals have equal energy.
Answer: Both A and R are true, but R is not the correct explanation.
Section G: Statement-Based Questions
1.
Statement I: Hund's Rule applies to p, d and f orbitals.
Statement II: These orbitals are degenerate.
Answer: Both statements are true.
2.
Statement I: Pairing begins after all degenerate orbitals are singly occupied.
Statement II: This reduces electron repulsion.
Answer: Both statements are true.
3.
Statement I: Half-filled orbitals are unstable.
Statement II: Fully filled orbitals are stable.
Answer: Statement I is false; Statement II is true.
4.
Statement I: Pairing in d orbitals begins with the sixth electron.
Statement II: A d subshell contains five orbitals.
Answer: Both statements are true.
5.
Statement I: Degenerate orbitals have equal energy.
Statement II: Hund's Rule explains their filling.
Answer: Both statements are true.
Section H: Match the Columns
| Column A | Column B |
|---|---|
| Hund's Rule | Single occupancy first |
| Degenerate orbitals | Equal energy |
| p subshell | 3 orbitals |
| d subshell | 5 orbitals |
| f subshell | 7 orbitals |
Answers
1 → Single occupancy first
2 → Equal energy
3 → 3 orbitals
4 → 5 orbitals
5 → 7 orbitals
Section I: Case Study Questions
Case Study
A student fills the three p orbitals in the following order:
↑ ↑ ↑
His friend asks why he did not pair the electrons first.
The teacher explains that this arrangement follows Hund's Rule.
Answer the following questions.
Q1. Which rule is followed?
Answer: Hund's Rule of Maximum Multiplicity.
Q2. What are degenerate orbitals?
Answer: Orbitals having the same energy.
Q3. Why are electrons placed singly first?
Answer: To reduce electron repulsion and increase stability.
Q4. When does pairing begin in p orbitals?
Answer: With the fourth electron.
Q5. Name one stable half-filled configuration.
Answer: p³ or d⁵.
Important CBSE Questions
- State Hund's Rule of Maximum Multiplicity.
- Define degenerate orbitals with examples.
- Explain the filling of electrons in p orbitals using Hund's Rule.
- Why are half-filled and fully filled subshells more stable?
- Explain why pairing starts only after single occupancy.
- Draw the electronic arrangement for p³ and p⁴.
- State the significance of Hund's Rule in writing electronic configurations.
- Differentiate between Hund's Rule and the Pauli Exclusion Principle.
- Explain the filling of d orbitals according to Hund's Rule.
- Why does Hund's Rule lead to greater atomic stability?
Complete Chapter Quick Revision (Very Important for CBSE & NEET)
| Principle | Main Rule | Key Point |
|---|---|---|
| Aufbau Principle | Lowest-energy orbitals fill first | 4s fills before 3d |
| Pauli Exclusion Principle | One orbital holds a maximum of 2 electrons | Spins must be opposite (↑↓) |
| Hund's Rule | Electrons occupy degenerate orbitals singly before pairing | Pairing starts only after single occupancy |
This completes the full CBSE Class 11 question bank for all three topics:
- Aufbau Principle
- Pauli Exclusion Principle
- Hund's Rule of Maximum Multiplicity
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