📘 Detailed Solution
🔵 Step 1: Atomic Number of Chromium
Chromium has atomic number Z = 24. Therefore, a neutral chromium atom contains 24 electrons.
🟢 Step 2: Expected Configuration
According to the simple Aufbau filling order, the expected configuration would be:
However, the actual ground-state configuration of chromium is:
🎬 Electron Arrangement
Expected:
4s² 3d⁴One 4s electron shifts to 3d
Actual stable arrangement:
4s¹ 3d⁵🟣 Step 3: Stability of Half-Filled d-Subshell
A half-filled d-subshell (d⁵) has one electron in each of the five d-orbitals. This arrangement is especially stable.
The five electrons occupy the five d-orbitals singly according to Hund's rule.
This arrangement gives additional stability because of factors such as exchange energy and a more symmetrical distribution of electrons.
🟠 Step 4: Exchange Energy
The half-filled d⁵ configuration allows a greater number of possible electron exchanges between electrons having parallel spins. This increases the exchange energy, which contributes to the stability of the atom.
🟢 Step 5: Final Explanation
One electron from the 4s orbital is promoted to the 3d orbital because the resulting 3d⁵ 4s¹ arrangement is more stable than 3d⁴ 4s².
✅ Final Answer
The electronic configuration of chromium is:
instead of [Ar] 3d⁴ 4s² because the half-filled 3d⁵ subshell is more stable. The extra stability is mainly due to exchange energy and symmetrical distribution of electrons.
📝 20 MCQs with Answers
1. The atomic number of chromium is:
✅ Answer
C) 24
2. The actual electronic configuration of chromium is:
✅ Answer
B) [Ar] 3d⁵ 4s¹
3. The expected configuration of chromium based on simple Aufbau filling is:
✅ Answer
B) [Ar] 3d⁴ 4s²
4. Which configuration is especially stable?
✅ Answer
C) d⁵
5. Chromium is an example of:
✅ Answer
A) Aufbau exception
6. A d⁵ configuration is called:
✅ Answer
B) Half-filled
7. The d-subshell contains how many orbitals?
✅ Answer
B) 5
8. Maximum electrons in a d-subshell are:
✅ Answer
C) 10
9. The stability of chromium is enhanced by:
✅ Answer
B) Exchange energy
10. In Cr, how many electrons are present in the 3d subshell?
✅ Answer
C) 5
11. In Cr, how many electrons are present in the 4s subshell?
✅ Answer
B) 1
12. Chromium belongs to the:
✅ Answer
C) d-block
13. The half-filled configuration of Cr is:
✅ Answer
C) 3d⁵
14. Which rule explains the distribution of electrons in degenerate d-orbitals?
✅ Answer
A) Hund's rule
15. Chromium has how many unpaired electrons in its ground state?
✅ Answer
C) 5
16. Which element also shows an exceptional configuration similar to Cr?
✅ Answer
A) Cu
17. The fully filled d-subshell is:
✅ Answer
D) d¹⁰
18. Chromium's exceptional configuration provides:
✅ Answer
A) Greater stability
19. The correct orbital representation of Cr's 3d⁵ electrons is:
✅ Answer
B) ↑ ↑ ↑ ↑ ↑
20. The best explanation for Cr configuration is:
✅ Answer
A) d⁵ half-filled stability
Q1. Write the actual electronic configuration of chromium.
Solution: Chromium (Z = 24) has the configuration [Ar] 3d⁵ 4s¹.
Q2. Why is 3d⁵ more stable than 3d⁴?
Solution: 3d⁵ is a half-filled d-subshell. It has greater exchange energy and a symmetrical distribution of electrons, giving additional stability.
Q3. What is meant by a half-filled d-subshell?
Solution: A d-subshell contains five orbitals. When each orbital contains one electron, the configuration is d⁵ and is called half-filled.
Q1. Explain the anomalous electronic configuration of chromium.
Solution:
- Expected configuration: [Ar] 3d⁴ 4s².
- One 4s electron shifts to the 3d subshell.
- The actual configuration becomes [Ar] 3d⁵ 4s¹, which is more stable due to half-filled d-subshell stability.
Q2. State the role of exchange energy in the stability of chromium.
Solution: In the 3d⁵ arrangement, five d-orbitals contain parallel-spin electrons. This permits greater exchange interaction, increasing exchange energy and thereby lowering the energy of the atom.
Q1. Compare the expected and actual electronic configurations of chromium and explain the difference.
| Expected | Actual |
|---|---|
| [Ar] 3d⁴ 4s² | [Ar] 3d⁵ 4s¹ |
| 3d subshell is less stable | 3d⁵ is half-filled and more stable |
| Two electrons in 4s | One electron transferred from 4s to 3d |
The actual configuration has lower overall energy because the half-filled 3d⁵ subshell receives additional stability.
Q1. Explain the anomalous electronic configuration of chromium using Aufbau principle, Hund's rule and the concept of exchange energy.
Solution:
- Chromium has atomic number 24.
- Simple Aufbau filling predicts [Ar] 3d⁴ 4s².
- However, one 4s electron moves to the 3d subshell.
- This produces [Ar] 3d⁵ 4s¹.
- The five 3d orbitals are singly occupied according to Hund's rule.
- The half-filled d⁵ arrangement has additional stability because of exchange energy and symmetrical electron distribution.
Q1. Give a detailed explanation of the electronic configuration of Chromium (Z = 24). Why does it differ from the expected Aufbau configuration?
Solution:
1️⃣ Atomic Number:
2️⃣ Expected configuration:
This follows the usual filling order, but it is not the lowest-energy arrangement for chromium.
3️⃣ Actual configuration:
One electron from the 4s orbital occupies the 3d orbital.
4️⃣ Half-filled d-subshell:
All five d-orbitals contain one electron each. This is a particularly stable arrangement.
5️⃣ Exchange Energy:
The 3d⁵ arrangement has maximum parallel-spin electrons and therefore greater exchange energy. Greater exchange energy contributes to lowering the energy and increasing stability.
6️⃣ Conclusion:
🎯 Quick Revision
Expected: [Ar] 3d⁴ 4s²
Actual: [Ar] 3d⁵ 4s¹
Reason: Half-filled d⁵ stability
Important Factors: Exchange Energy + Symmetrical Distribution
