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

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2. Which subshell is progressively filled in Lanthanoids?

A) 3d

B) 4f

C) 5f

D) 4d

✅ Answer

B) 4f

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3. Which subshell is progressively filled in Actinoids?

A) 4f

B) 4d

C) 5f

D) 5d

✅ Answer

C) 5f

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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

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5. Lanthanoids are mainly associated with:

A) 3d series

B) 4f series

C) 5f series

D) 6d series

✅ Answer

B) 4f series

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6. Actinoids are mainly associated with:

A) 4f series

B) 4d series

C) 5f series

D) 6p series

✅ Answer

C) 5f series

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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

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8. Lanthanoids belong mainly to which period?

A) 4

B) 5

C) 6

D) 7

✅ Answer

C) Period 6

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9. Actinoids belong mainly to which period?

A) 5

B) 6

C) 7

D) 8

✅ Answer

C) Period 7

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

A) 2

B) 6

C) 10

D) 14

✅ Answer

D) 14

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

A) 3

B) 5

C) 7

D) 9

✅ Answer

C) 7

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12. Which series includes Uranium?

A) Lanthanoids

B) Actinoids

C) Halogens

D) Transition metals

✅ Answer

B) Actinoids

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13. Which series includes Cerium?

A) Lanthanoids

B) Actinoids

C) d-block

D) p-block

✅ Answer

A) Lanthanoids

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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

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15. Which is a Lanthanoid?

A) U

B) Ce

C) Th

D) Pu

✅ Answer

B) Ce

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16. Which is an Actinoid?

A) Nd

B) Ce

C) U

D) Eu

✅ Answer

C) U

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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

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18. Lanthanoid contraction is associated with:

A) 4f electrons

B) 5p electrons

C) 3d electrons

D) 6p electrons

✅ Answer

A) 4f electrons

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19. Most Actinoids are:

A) Non-metals

B) Noble gases

C) Radioactive

D) Halogens

✅ Answer

C) Radioactive

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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