Explain why the electronic configuration of Chromium (Z=24) is 4s 1 3d 5 instead of 4s 2 3d 4 .

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Solution

🔵 Step 1: Atomic Number of Chromium

Chromium has atomic number Z = 24. Therefore, a neutral chromium atom contains 24 electrons.

Z = 24 → Cr → 24 electrons

🟢 Step 2: Expected Configuration

According to the simple Aufbau filling order, the expected configuration would be:

[Ar] 3d⁴ 4s²

However, the actual ground-state configuration of chromium is:

[Ar] 3d⁵ 4s¹

🎬 Electron Arrangement

Expected:

4s² 3d⁴

↓

One 4s electron shifts to 3d

⚡ → 🟢

Actual stable arrangement:

4s¹ 3d⁵

Half-filled 3d⁵ subshell → Greater Stability

🟣 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.

3d⁵ = ↑   ↑   ↑   ↑   ↑

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.

Higher Exchange Energy → Greater Stability

🟢 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:

[Ar] 3d⁵ 4s¹

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:

A) 22

B) 23

C) 24

D) 25

✅ Answer

C) 24

---

2. The actual electronic configuration of chromium is:

A) [Ar] 3d⁴ 4s²

B) [Ar] 3d⁵ 4s¹

C) [Ar] 3d³ 4s³

D) [Ar] 3d⁶

✅ Answer

B) [Ar] 3d⁵ 4s¹

---

3. The expected configuration of chromium based on simple Aufbau filling is:

A) [Ar] 3d⁵ 4s¹

B) [Ar] 3d⁴ 4s²

C) [Ar] 3d⁶

D) [Ne] 3d⁶ 4s²

✅ Answer

B) [Ar] 3d⁴ 4s²

---

4. Which configuration is especially stable?

A) d¹

B) d²

C) d⁵

D) d⁶

✅ Answer

C) d⁵

---

5. Chromium is an example of:

A) Aufbau exception

B) Noble gas

C) Halogen

D) s-block element

✅ Answer

A) Aufbau exception

---

6. A d⁵ configuration is called:

A) Completely filled

B) Half-filled

C) Empty

D) Doubly filled

✅ Answer

B) Half-filled

---

7. The d-subshell contains how many orbitals?

A) 3

B) 5

C) 7

D) 10

✅ Answer

B) 5

---

8. Maximum electrons in a d-subshell are:

A) 2

B) 6

C) 10

D) 14

✅ Answer

C) 10

---

9. The stability of chromium is enhanced by:

A) Pairing energy only

B) Exchange energy

C) Nuclear decay

D) Ionisation only

✅ Answer

B) Exchange energy

---

10. In Cr, how many electrons are present in the 3d subshell?

A) 3

B) 4

C) 5

D) 6

✅ Answer

C) 5

---

11. In Cr, how many electrons are present in the 4s subshell?

A) 0

B) 1

C) 2

D) 3

✅ Answer

B) 1

---

12. Chromium belongs to the:

A) s-block

B) p-block

C) d-block

D) f-block

✅ Answer

C) d-block

---

13. The half-filled configuration of Cr is:

A) 3d³

B) 3d⁴

C) 3d⁵

D) 3d¹⁰

✅ Answer

C) 3d⁵

---

14. Which rule explains the distribution of electrons in degenerate d-orbitals?

A) Hund's rule

B) Boyle's law

C) Charles' law

D) Raoult's law

✅ Answer

A) Hund's rule

---

15. Chromium has how many unpaired electrons in its ground state?

A) 1

B) 2

C) 4

D) 6

✅ Answer

C) 5

---

16. Which element also shows an exceptional configuration similar to Cr?

A) Cu

B) Ca

C) Ne

D) Mg

✅ Answer

A) Cu

---

17. The fully filled d-subshell is:

A) d³

B) d⁵

C) d⁸

D) d¹⁰

✅ Answer

D) d¹⁰

---

18. Chromium's exceptional configuration provides:

A) Greater stability

B) Less stability

C) No change

D) Radioactivity

✅ Answer

A) Greater stability

---

19. The correct orbital representation of Cr's 3d⁵ electrons is:

A) ↑↓ ↑↓ ↑

B) ↑ ↑ ↑ ↑ ↑

C) ↑↓ ↑ ↑ ↑

D) ↑↓ ↑↓ ↑↓

✅ Answer

B) ↑ ↑ ↑ ↑ ↑

---

20. The best explanation for Cr configuration is:

A) d⁵ half-filled stability

B) d⁴ is more stable

C) 4s² is always stable

D) Cr has no d-electrons

✅ 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:

1. Expected configuration: [Ar] 3d⁴ 4s².
2. One 4s electron shifts to the 3d subshell.
3. 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:

1. Chromium has atomic number 24.
2. Simple Aufbau filling predicts [Ar] 3d⁴ 4s².
3. However, one 4s electron moves to the 3d subshell.
4. This produces [Ar] 3d⁵ 4s¹.
5. The five 3d orbitals are singly occupied according to Hund's rule.
6. The half-filled d⁵ arrangement has additional stability because of exchange energy and symmetrical electron distribution.

Therefore, [Ar] 3d⁵ 4s¹ is energetically more stable than [Ar] 3d⁴ 4s².

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:

Cr = 24 electrons

2️⃣ Expected configuration:

[Ar] 3d⁴ 4s²

This follows the usual filling order, but it is not the lowest-energy arrangement for chromium.

3️⃣ Actual configuration:

[Ar] 3d⁵ 4s¹

One electron from the 4s orbital occupies the 3d orbital.

4️⃣ Half-filled d-subshell:

3d⁵ → ↑ ↑ ↑ ↑ ↑

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:

The actual electronic configuration of chromium is [Ar] 3d⁵ 4s¹, not [Ar] 3d⁴ 4s², because the half-filled 3d⁵ subshell provides additional stability through exchange energy and symmetrical distribution of electrons.

🎯 Quick Revision

Cr: Z = 24
Expected: [Ar] 3d⁴ 4s²
Actual: [Ar] 3d⁵ 4s¹
Reason: Half-filled d⁵ stability
Important Factors: Exchange Energy + Symmetrical Distribution

Which block do the Lanthanoids and Actinoids belong to?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Solution

🔵 Step 1: Identify the elements

The Lanthanoids and Actinoids are two series of elements shown separately at the bottom of the modern periodic table.

Lanthanoids + Actinoids → f-block elements

🟢 Step 2: Why are they called f-block elements?

They are called f-block elements because the differentiating electron enters the f-subshell.

(n − 2)f subshell is progressively filled

Therefore, these elements are placed in the f-block of the periodic table.

🎬 f-Block Electronic Filling

La Ce Pr Nd

↓

4f subshell filling

Ac Th Pa U

↓

5f subshell filling

🟣 Lanthanoids

The Lanthanoids involve the progressive filling of the 4f subshell.

Lanthanoids → 4f filling → f-block

🟠 Actinoids

The Actinoids involve the progressive filling of the 5f subshell.

Actinoids → 5f filling → f-block

📊 Quick Comparison

Series	Subshell Being Filled	Block
Lanthanoids	4f	f-block
Actinoids	5f	f-block

✅ Final Answer

Lanthanoids and Actinoids belong to the f-block of the periodic table because the differentiating electrons enter the f-subshell.

Lanthanoids → 4f
Actinoids → 5f

📝 MCQs with Answers

1. Lanthanoids and Actinoids belong to which block?

A) s-block

B) p-block

C) d-block

D) f-block

✅ Answer

D) f-block

---

2. Which subshell is progressively filled in Lanthanoids?

A) 3d

B) 4f

C) 5f

D) 4d

✅ Answer

B) 4f

---

3. Which subshell is progressively filled in Actinoids?

A) 4f

B) 4d

C) 5f

D) 5d

✅ Answer

C) 5f

---

4. The f-block elements are placed separately at the:

A) Top of the table

B) Bottom of the table

C) Right side only

D) Left side only

✅ Answer

B) Bottom of the table

---

5. Lanthanoids are mainly associated with:

A) 3d series

B) 4f series

C) 5f series

D) 6d series

✅ Answer

B) 4f series

---

6. Actinoids are mainly associated with:

A) 4f series

B) 4d series

C) 5f series

D) 6p series

✅ Answer

C) 5f series

---

7. The f-block elements are also called:

A) Inner transition elements

B) Noble gases

C) Alkali metals

D) Halogens

✅ Answer

A) Inner transition elements

---

8. Lanthanoids belong mainly to which period?

A) 4

B) 5

C) 6

D) 7

✅ Answer

C) Period 6

---

9. Actinoids belong mainly to which period?

A) 5

B) 6

C) 7

D) 8

✅ Answer

C) Period 7

---

10. The maximum number of electrons in an f-subshell is:

A) 2

B) 6

C) 10

D) 14

✅ Answer

D) 14

---

11. How many orbitals are present in an f-subshell?

A) 3

B) 5

C) 7

D) 9

✅ Answer

C) 7

---

12. Which series includes Uranium?

A) Lanthanoids

B) Actinoids

C) Halogens

D) Transition metals

✅ Answer

B) Actinoids

---

13. Which series includes Cerium?

A) Lanthanoids

B) Actinoids

C) d-block

D) p-block

✅ Answer

A) Lanthanoids

---

14. The f-block is called inner transition series because:

A) Inner f-orbitals are progressively filled

B) They have no electrons

C) They are gases

D) They belong to Group 18

✅ Answer

A) Inner f-orbitals are progressively filled

---

15. Which is a Lanthanoid?

A) U

B) Ce

C) Th

D) Pu

✅ Answer

B) Ce

---

16. Which is an Actinoid?

A) Nd

B) Ce

C) U

D) Eu

✅ Answer

C) U

---

17. The differentiating electron in f-block elements enters the:

A) s-subshell

B) p-subshell

C) d-subshell

D) f-subshell

✅ Answer

D) f-subshell

---

18. Lanthanoid contraction is associated with:

A) 4f electrons

B) 5p electrons

C) 3d electrons

D) 6p electrons

✅ Answer

A) 4f electrons

---

19. Most Actinoids are:

A) Non-metals

B) Noble gases

C) Radioactive

D) Halogens

✅ Answer

C) Radioactive

---

20. The correct pair is:

A) Lanthanoids – 5f

B) Actinoids – 4f

C) Lanthanoids – 4f and Actinoids – 5f

D) Both – 3d

✅ Answer

C) Lanthanoids – 4f and Actinoids – 5f

Q1. Which block do Lanthanoids and Actinoids belong to?

Solution: Both Lanthanoids and Actinoids belong to the f-block because the differentiating electrons enter f-orbitals.

Q2. Which subshell is filled in Lanthanoids and Actinoids?

Solution: Lanthanoids involve filling of the 4f subshell, while Actinoids involve filling of the 5f subshell.

Q3. Why are f-block elements called inner transition elements?

Solution: They are called inner transition elements because the differentiating electron enters an inner (n−2)f subshell.

Q1. Differentiate between Lanthanoids and Actinoids.

Lanthanoids	Actinoids
4f orbitals are progressively filled.	5f orbitals are progressively filled.
Belong to Period 6.	Belong to Period 7.
Generally less radioactive.	Most are radioactive.

Q2. Explain why f-block elements are placed separately at the bottom of the periodic table.

Solution: The f-block contains two long series of elements. Placing them within the main body would make the periodic table excessively wide. Therefore, they are conventionally shown separately at the bottom.

Q1. Explain the position and electronic structure of f-block elements.

Solution:

1. f-block elements are placed separately at the bottom of the periodic table.
2. They consist mainly of Lanthanoids and Actinoids.
3. The differentiating electron enters the f-subshell.
4. Lanthanoids involve 4f filling, while Actinoids involve 5f filling.

Both series belong to the f-block and are called inner transition elements.

Q1. Describe the f-block elements and distinguish between Lanthanoids and Actinoids.

Solution:

The f-block elements are those in which the differentiating electron enters an f-subshell. They are also known as inner transition elements.

Property	Lanthanoids	Actinoids
Subshell filled	4f	5f
Period	6	7
Nature	Mostly metallic	Metallic
Radioactivity	Generally not all radioactive	Mostly radioactive

Lanthanoids → 4f → Period 6
Actinoids → 5f → Period 7

Q1. Explain the f-block elements in detail. Discuss Lanthanoids, Actinoids, their electronic configuration and their position in the periodic table.

Solution:

The f-block consists of elements in which the differentiating electron enters an f-subshell. These elements are known as inner transition elements.

1️⃣ Lanthanoids

Lanthanoids are the elements of Period 6 in which the 4f subshell is progressively filled.

Lanthanoids → 4f filling → Period 6

2️⃣ Actinoids

Actinoids are the elements of Period 7 in which the 5f subshell is progressively filled.

Actinoids → 5f filling → Period 7

3️⃣ Position

Both series are shown separately at the bottom of the periodic table to keep the main periodic table compact.

4️⃣ Electronic Basis

The general characteristic of these elements is progressive filling of the inner f-subshell.

(n − 2)f^1–14

5️⃣ Important Difference

Feature	Lanthanoids	Actinoids
Subshell	4f	5f
Period	6	7
Examples	Ce, Pr, Nd	Th, Pa, U

6️⃣ Conclusion

Lanthanoids and Actinoids are f-block elements because their differentiating electrons enter the f-subshell. Lanthanoids involve 4f filling, whereas Actinoids involve 5f filling.

🎯 Quick Revision

Lanthanoids → 4f → f-block → Period 6
Actinoids → 5f → f-block → Period 7
Both → Inner Transition Elements

Determine the period, group, and block for an element with Z = 37.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Solution

🔵 Step 1: Identify the Element

Atomic number Z = 37 corresponds to the element Rubidium (Rb).

Z = 37 → Rubidium (Rb)

🟢 Step 2: Write the Electronic Configuration

The electronic configuration of Rubidium is:

Rb = [Kr] 5s¹

The complete configuration is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹

🎬 Electronic Configuration Animation

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹

↓

[Kr] 5s¹

🟣 Step 3: Determine the Period

The period is determined by the highest principal quantum number (n) present in the electronic configuration.

For Rb:

Highest n = 5
Therefore, Period = 5

🟠 Step 4: Determine the Group

Rubidium has one electron in its outermost shell:

5s¹

Elements having the outer configuration ns¹ belong to Group 1.

Group = 1

🟢 Step 5: Determine the Block

The last electron enters the s-subshell. Therefore, the element belongs to the s-block.

Block = s-block

✅ Final Answer

Z = 37 → Rubidium (Rb)

Period = 5
Group = 1
Block = s-block

📝 20 MCQs – Practice

1. The element with atomic number 37 is:

A) Potassium

B) Rubidium

C) Strontium

D) Caesium

✅ Answer

B) Rubidium

---

2. The symbol of element Z = 37 is:

A) Rb

B) Ru

C) Ra

D) Rh

✅ Answer

A) Rb

---

3. The electronic configuration of Rb is:

A) [Kr] 5s¹

B) [Ar] 4s¹

C) [Kr] 4d¹

D) [Xe] 6s¹

✅ Answer

A) [Kr] 5s¹

---

4. Rb belongs to which period?

A) 3

B) 4

C) 5

D) 6

✅ Answer

C) 5

---

5. Rb belongs to which group?

A) 1

B) 2

C) 17

D) 18

✅ Answer

A) 1

---

6. Rb belongs to which block?

A) p-block

B) d-block

C) s-block

D) f-block

✅ Answer

C) s-block

---

7. The last electron of Rb enters:

A) 4p

B) 5s

C) 4d

D) 5p

✅ Answer

B) 5s

---

8. The valence-shell configuration of Rb is:

A) 5s¹

B) 5s²

C) 5p¹

D) 4d¹

✅ Answer

A) 5s¹

---

9. The highest principal quantum number in Rb is:

A) 3

B) 4

C) 5

D) 6

✅ Answer

C) 5

---

10. Group 1 elements generally have outer configuration:

A) ns¹

B) ns²

C) ns²np⁵

D) ns²np⁶

✅ Answer

A) ns¹

---

11. Which element is in the same group as Rb?

A) Ca

B) K

C) Br

D) Kr

✅ Answer

B) K

---

12. Rb is a:

A) Noble gas

B) Alkali metal

C) Halogen

D) Transition metal

✅ Answer

B) Alkali metal

---

13. Which block is determined by the subshell receiving the differentiating electron?

A) s-block

B) p-block

C) d-block

D) All of these

✅ Answer

D) All of these

---

14. The atomic number immediately before Rb is:

A) 35

B) 36

C) 38

D) 39

✅ Answer

B) 36

---

15. Atomic number 36 represents:

A) Kr

B) Rb

C) Sr

D) Br

✅ Answer

A) Kr

---

16. Rb has how many valence electrons?

A) 1

B) 2

C) 7

D) 8

✅ Answer

A) 1

---

17. Rb is located in the:

A) Leftmost part of the periodic table

B) Rightmost part

C) Middle d-block

D) f-block

✅ Answer

A) Leftmost part

---

18. Which orbital is being filled in Rb?

A) 4s

B) 4p

C) 5s

D) 3d

✅ Answer

C) 5s

---

19. Rb has one electron outside the:

A) [Ne] core

B) [Ar] core

C) [Kr] core

D) [Xe] core

✅ Answer

C) [Kr] core

---

20. The correct classification for Z = 37 is:

A) Period 4, Group 2, p-block

B) Period 5, Group 1, s-block

C) Period 5, Group 2, s-block

D) Period 4, Group 1, d-block

✅ Answer

B) Period 5, Group 1, s-block

Q1. Identify the element with Z = 37 and write its electronic configuration.

Solution: Z = 37 is Rubidium (Rb). Its configuration is [Kr] 5s¹.

Q2. What is the group and block of Rb?

Solution: Rb has outer configuration 5s¹. Therefore, it belongs to Group 1 and s-block.

Q3. Determine the period of Rb.

Solution: The highest value of n is 5 in 5s¹. Hence, Rb belongs to Period 5.

Q1. Determine the period, group and block of Z = 37.

Solution:

1. Z = 37 → Rb.
2. Electronic configuration = [Kr] 5s¹.
3. Highest n = 5 → Period 5; ns¹ → Group 1; last electron enters s-subshell → s-block.

Period = 5 | Group = 1 | Block = s-block

Q2. Explain how electronic configuration helps in locating Rb in the periodic table.

Solution: The highest shell number gives the period. The outermost 5s¹ configuration gives Group 1, and the differentiating electron enters the s-subshell, giving s-block.

Q1. Write the complete electronic configuration of Z = 37 and determine its position in the periodic table.

Solution:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹

Highest n = 5, therefore Period 5. The valence configuration is 5s¹, therefore Group 1. The last electron enters an s-orbital, therefore s-block.

Rb → Period 5 → Group 1 → s-block

Q1. Explain systematically how the position of an element with atomic number 37 is determined in the periodic table.

Solution:

1. Atomic number 37 corresponds to Rubidium (Rb).
2. Its electronic configuration is [Kr] 5s¹.
3. The highest principal quantum number is 5, so it belongs to Period 5.
4. The outermost configuration is ns¹, so it belongs to Group 1.
5. The differentiating electron enters the s-subshell, so it belongs to the s-block.

Final Position: Period 5, Group 1, s-block

Q1. For an element having atomic number 37, determine its electronic configuration, element name, period, group and block. Explain each step.

Solution:

Step 1 – Identify the element:

Z = 37 → Rubidium (Rb)

Step 2 – Electronic configuration:

Rb = [Kr] 5s¹

Step 3 – Determine the period:

The highest principal quantum number in the configuration is n = 5. Therefore, the element belongs to Period 5.

Step 4 – Determine the group:

The outermost configuration is 5s¹. Elements with ns¹ configuration belong to Group 1.

Step 5 – Determine the block:

The differentiating electron enters the s-subshell. Therefore, the element belongs to the s-block.

Step 6 – Final classification:

Z = 37 → Rb

Period = 5
Group = 1
Block = s-block
Type = Alkali Metal

🎯 Quick Revision

Z = 37
Element = Rubidium (Rb)
Configuration = [Kr] 5s¹
Period = 5
Group = 1
Block = s-block