Stability of Half-Filled and Fully Filled Subshells | NEET Class 11 Chemistry Notes

Exchange Energy & Electronic Configuration Stability | Class 11 Chemistry 

Why half-filled (d⁵) and fully filled (d¹⁰) subshells are extra stable due to symmetry and exchange energy.

- Dr.Sanjaykumar Pawar 

---

🌟 Stability of Completely Filled and Half-Filled Subshells

📌 1. Ground State Idea

• The ground state electronic configuration means the most stable arrangement of electrons.
• In ground state, electrons have lowest possible total energy.
• Nature always prefers maximum stability → minimum energy.

---

📌 2. Why half-filled and completely filled subshells are stable?

There are two main reasons:

---

⭐ (1) Symmetrical Distribution of Electrons

• Symmetry = more stability in nature.
• In completely filled subshells (like d¹⁰) or half-filled subshells (like d⁵):
  • Electrons are evenly distributed in all orbitals.
• This arrangement is very balanced and stable.

---

📌 What happens in same subshell?

• Orbitals in the same subshell (like 3d) have:
  • Same energy (degenerate orbitals)
  • Different positions in space
• Because of this:
  • Shielding between electrons is very small
  • Attraction between nucleus and electrons becomes stronger
• This increases stability.

---

⭐ (2) Exchange Energy

• Electrons with same spin in degenerate orbitals can exchange positions.
• This swapping releases energy → called exchange energy.

---

📌 Important points:

• More exchanges = more stability
• Maximum exchanges happen when:
  • Subshell is half-filled
  • OR completely filled

---

📌 Result:

• Exchange energy becomes maximum
• So stability also becomes maximum

---

📌 Link with Hund’s Rule

• Hund’s rule says:
  • Electrons occupy orbitals singly first
  • And with parallel spins
• Reason: to maximize exchange energy and stability

---

⭐ Extra Stability Reasons (NEET Important)

Half-filled and fully filled subshells are more stable because:

1. ✔ Less electron shielding
2. ✔ Lower electron-electron repulsion (coulombic repulsion)
3. ✔ Higher exchange energy

---

📌 3. Special Cases: Chromium (Cr) and Copper (Cu)

Sometimes exceptions occur because:

• 4s and 3d subshells have very close energy levels
• Electrons shift to achieve stability

---

⭐ Chromium (Cr)

• Expected: 3d⁴ 4s² ❌
• Actual: 3d⁵ 4s¹ ✔

👉 Reason:

• 3d⁵ is half-filled → extra stable

---

⭐ Copper (Cu)

• Expected: 3d⁹ 4s² ❌
• Actual: 3d¹⁰ 4s¹ ✔

👉 Reason:

• 3d¹⁰ is completely filled → extra stable

---

📌 Final Key Idea (NEET Revision Line)

• Half-filled and fully filled subshells are extra stable due to:
  • Symmetry
  • Maximum exchange energy
  • Minimum repulsion

---

STABILITY OF SUBSHELLS

│

├── 1. Ground State Concept

│ ├── Lowest energy state of atom

│ └── Most stable electronic configuration

│

├── 2. Why half-filled & completely filled subshells are stable?

│

│ ├── (A) Symmetrical Distribution

│ │ ├── Electrons evenly arranged in orbitals

│ │ ├── Half-filled (d⁵) / Fully filled (d¹⁰)

│ │ ├── Less shielding between electrons

│ │ └── Stronger attraction to nucleus

│ │

│ ├── (B) Exchange Energy

│ │ ├── Electrons with same spin can exchange positions

│ │ ├── Exchange releases energy → stability increases

│ │ ├── Maximum exchange in:

│ │ │ ├── Half-filled subshell

│ │ │ └── Completely filled subshell

│ │ └── More exchange = more stability

│ │

│ └── (C) Other Stability Factors

│ ├── Less electron-electron repulsion

│ ├── Less shielding effect

│ └── Higher exchange energy

│

├── 3. Link with Hund’s Rule

│ ├── Electrons occupy orbitals singly first

│ ├── Parallel spins preferred

│ └── Reason: maximum exchange energy

│

├── 4. Exceptions in Electronic Configuration

│

│ ├── Chromium (Cr)

│ │ ├── Expected: 3d⁴ 4s²

│ │ └── Actual: 3d⁵ 4s¹ (half-filled stability)

│ │

│ └── Copper (Cu)

│ ├── Expected: 3d⁹ 4s²

│ └── Actual: 3d¹⁰ 4s¹ (fully filled stability)

│

└── 5. Final Conclusion

      ├── Half-filled subshell → extra stable

      ├── Fully filled subshell → extra stable

      └── Reason = symmetry + exchange energy + low repulsion 

🌟 VERY SHORT ANSWER QUESTIONS (1 MARK)

Q1. What is ground state?

Ans: The lowest energy state of an atom.

---

Q2. Why are half-filled subshells stable?

Ans: Due to symmetry and maximum exchange energy.

---

Q3. What is exchange energy?

Ans: Energy released due to exchange of electrons with same spin in degenerate orbitals.

---

Q4. Which subshell is more stable: d⁵ or d⁴?

Ans: d⁵ is more stable.

---

Q5. Write Cu ground state configuration.

Ans: 3d¹⁰ 4s¹

---

🌟 SHORT ANSWER QUESTIONS (2–3 MARKS)

Q1. State two reasons for extra stability of half-filled subshells.

Ans:

1. Symmetrical distribution of electrons.
2. Maximum exchange energy.

---

Q2. Why is Cr exception in electronic configuration?

Ans: Because half-filled 3d⁵ is more stable than 3d⁴ 4s², so one electron shifts from 4s to 3d.

---

Q3. What is Hund’s rule based on?

Ans: It is based on maximizing exchange energy by keeping electrons unpaired with parallel spins in degenerate orbitals.

---

🌟 LONG ANSWER QUESTIONS (5 MARKS)

Q1. Explain stability of half-filled and completely filled subshells.

Ans: Half-filled and completely filled subshells are more stable due to:

1. Symmetry: Electrons are evenly distributed in orbitals, reducing repulsion.
2. Exchange energy: More possible exchanges between electrons of same spin increases stability.
3. Lower repulsion: Electron-electron repulsion is less in stable configurations.
4. Lower shielding effect: Electrons experience stronger attraction from nucleus.
5. Result: Half-filled (d⁵) and fully filled (d¹⁰) are extra stable.

---

Q2. Explain Cr and Cu exceptions.

Ans:

• Chromium expected: 3d⁴ 4s² but actual is 3d⁵ 4s¹
• Copper expected: 3d⁹ 4s² but actual is 3d¹⁰ 4s¹

Reason:

• Half-filled and fully filled subshells are more stable.
• Electron shifts from 4s to 3d to gain stability.

---

🌟 ASSERTION AND REASON

Q1.

A: Half-filled subshells are more stable.
R: They have maximum exchange energy.

✔ Answer: Both A and R are true, and R is correct explanation.

---

Q2.

A: Cu has configuration 3d⁹ 4s².
R: Fully filled subshell is more stable.

✔ Answer: A is false, R is true.

---

Q3.

A: Exchange energy increases stability.
R: It is maximum in half-filled and fully filled subshells.

✔ Answer: Both true, R explains A.

---

🌟 FILL IN THE BLANKS

1. Ground state has ______ energy.
   ✔ Lowest

2. Exchange energy is maximum in ______ subshells.
   ✔ half-filled and completely filled

3. Chromium has configuration ______.
   ✔ 3d⁵ 4s¹

4. Stability is due to symmetry and ______ energy.
   ✔ exchange

5. Cu has configuration ______.
   ✔ 3d¹⁰ 4s¹

---

🌟 MATCH THE COLUMN

Column A	Column B
Cr	3d⁵ 4s¹
Cu	3d¹⁰ 4s¹
Exchange energy	Stability
Hund’s rule	Parallel spins
Half-filled subshell	d⁵

---

🌟 CASE STUDY QUESTION

Passage:

Electrons in degenerate orbitals tend to remain unpaired and have parallel spins. This arrangement increases exchange energy and stability. Some elements show exceptions like Cr and Cu due to extra stability of half-filled and fully filled subshells.

---

Questions:

Q1. Which rule explains electron arrangement in orbitals?
Ans: Hund’s rule

---

Q2. Why is exchange energy important?
Ans: It increases stability of atom.

---

Q3. Why does Cr show exception?
Ans: To achieve half-filled stable configuration (3d⁵).

---

Q4. What is Cu configuration?
Ans: 3d¹⁰ 4s¹

---

Q5. What is the main reason of stability?
Ans: Exchange energy and symmetry.

---

🌟 STATEMENT QUESTIONS

Q1.

Statement 1: Half-filled subshells are stable.
Statement 2: They have maximum exchange energy.

✔ Both correct, Statement 2 explains Statement 1.

---

Q2.

Statement 1: Cu is 3d⁹ 4s²
Statement 2: Stability is achieved in fully filled subshells

✔ Statement 1 is false, Statement 2 is true.

---

🌟 INTERNAL LINKS

/class11/chemistry/atomic-structure

/neet/chemistry/electronic-configuration

/chemistry/hunds-rule-exchange-energy

/neet/important-exceptions-cr-cu

/class11/chemistry/orbital-theory-basics