FORCE

 

CLASS 12 CHEMISTRY JEE MAIN REVISION

f-Block Elements: Complete Notes, Trends & MCQs

Learn lanthanoids, actinoids, electronic configurations, oxidation states, contraction and important exam facts.

Quick Revision • Important Concepts • Practice Questions

The f-block elements are an important topic in Class 12 Inorganic Chemistry and JEE Main. Questions commonly focus on electronic configuration, lanthanoid contraction, oxidation states, magnetic properties and the differences between lanthanoids and actinoids.

Learning objectives

• Understand the position and classification of f-block elements.
• Learn their electronic configurations and oxidation states.
• Understand lanthanoid contraction and its consequences.
• Revise important facts and solve exam-style MCQs.

📚 Table of Contents

1. Introduction to f-block elements
2. Electronic configuration
3. Lanthanoids
4. Actinoids
5. Lanthanoids vs Actinoids
6. Lanthanoid contraction
7. Important properties
8. JEE Main quick revision
9. Practice questions with answers

1. What Are f-Block Elements?

Elements in which the differentiating electron enters an f-subshell are called f-block elements. They are shown separately at the bottom of the periodic table to keep the table compact.

General electronic configuration:
(n − 2)f^0–14 (n − 1)d^0–1 ns²

The f-block contains two series:

🔵 Lanthanoids

The 4f series, commonly discussed from cerium (Ce, Z = 58) to lutetium (Lu, Z = 71). Lanthanum (La, Z = 57) is closely associated with the series and is often included in school-level discussions of lanthanoids.

🟣 Actinoids

The 5f series, commonly discussed from thorium (Th, Z = 90) to lawrencium (Lr, Z = 103). Actinium (Ac, Z = 89) is closely associated with the series and is commonly included in school-level discussions of actinoids.

Exam tip: Do not confuse the f-block with the d-block. In the d-block, the differentiating electron enters a d-subshell; in the f-block, it enters an f-subshell.

2. Electronic Configuration

The two series involve the progressive filling of 4f and 5f orbitals, respectively.

Series	Subshell being filled	Associated period
Lanthanoids	4f	Period 6
Actinoids	5f	Period 7

Important: The general configuration is a guide, not an exception-free rule. The energies of the ns, (n − 1)d and (n − 2)f orbitals are close, so some elements have configurations that differ from a simple filling pattern.

3. Lanthanoids

Lanthanoids are the elements associated with the filling of the 4f subshell. They are generally silvery metals and show many similar chemical properties.

Important characteristics

• Common oxidation state: +3.
• Other oxidation states: +2 and +4 occur for some elements, such as Eu(II), Yb(II), Ce(IV) and Tb(IV), in suitable compounds.
• Colour: Many lanthanoid ions are coloured because of f–f electronic transitions.
• Magnetism: Ions with unpaired electrons are generally paramagnetic.
• Chemical similarity: Most lanthanoids form +3 ions and have similar ionic radii and chemical behaviour.

Remember: La³⁺ has a 4f⁰ configuration, while Lu³⁺ has 4f¹⁴. Both have no unpaired 4f electrons and are diamagnetic.

4. Actinoids

Actinoids are the elements associated with the filling of the 5f subshell. The 5f, 6d and 7s orbital energies are close, leading to more complicated electronic configurations and oxidation states.

Important characteristics

• Radioactivity: All actinoids are radioactive.
• Oxidation states: They show greater variation than lanthanoids. Oxidation states from +3 to +7 occur among different actinoids, although not every element exhibits every state.
• Complex formation: Actinoids often form complexes and show more varied bonding behaviour.
• Contraction: Actinoid contraction occurs across the series because the effective nuclear charge increases while 5f electrons shield imperfectly.
• Occurrence: Several actinoids are synthetic; some, such as uranium and thorium, occur naturally in significant amounts.

JEE Main fact: Uranium (U, Z = 92) is an actinoid. Its compounds are important in nuclear chemistry. Do not assume that all actinoids are synthetic; uranium and thorium are naturally occurring examples.

5. Lanthanoids vs Actinoids

Property	Lanthanoids	Actinoids
Subshell filled	4f	5f
Period	6	7
Common oxidation state	+3	+3, with more variable states
Radioactivity	Most are not radioactive; promethium has no stable isotopes	All are radioactive
Complex formation	Generally less extensive	Often more extensive
Bonding	4f electrons usually participate little in bonding	5f electrons may participate more in bonding
Contraction	Lanthanoid contraction	Actinoid contraction

6. Lanthanoid Contraction

Definition: Lanthanoid contraction is the gradual decrease in the atomic and especially the Ln³⁺ ionic radii across the lanthanoid series as atomic number increases.

Cause

As the nuclear charge increases, the added 4f electrons shield the outer electrons imperfectly. The effective nuclear attraction therefore increases, pulling the electron cloud closer to the nucleus.

Consequences of lanthanoid contraction

1. Similarity of 4d and 5d transition elements

Zr and Hf have very similar atomic and ionic radii. This similarity is largely associated with lanthanoid contraction.

2. Difficulty in separation

Lanthanoids have similar ionic radii and mostly form +3 ions, making their chemical separation difficult.

3. Decrease in ionic size

The Ln³⁺ ionic radius generally decreases from the beginning to the end of the series.

Memory trick: Higher atomic number + poor 4f shielding → greater effective nuclear attraction → smaller ionic radius.

7. Important Chemical and Physical Properties

A. Oxidation states

The +3 oxidation state is common among lanthanoids because the loss of two 6s electrons and one additional electron leads to a particularly common ionic state.

Ion	Configuration clue	Exam point
Ce4+	4f0	Empty 4f subshell
Eu2+	4f7	Half-filled 4f subshell
Yb2+	4f14	Completely filled 4f subshell
Gd3+	4f7	Half-filled 4f subshell

B. Colour of ions

Many lanthanoid ions are coloured due to electronic transitions within the 4f subshell. The colour depends on the ion and its electronic structure.

Ions with 4f⁰ or 4f¹⁴ configurations generally lack f–f transitions and are commonly colourless in aqueous solution.

C. Magnetic properties

Unpaired electrons contribute to paramagnetism. Ions with all electrons paired are diamagnetic.

Spin-only approximation: μ = √[n(n + 2)] BM

Here, n is the number of unpaired electrons and BM stands for Bohr magneton. This formula is a useful approximation for many transition-metal ions; lanthanoid magnetic moments generally require consideration of orbital angular momentum as well.

D. Formation of oxides and hydroxides

Lanthanoids form oxides and hydroxides. Their basic character generally decreases across the series as the ionic size decreases and charge density increases.

8. JEE Main Quick Revision Sheet

• f-block: differentiating electron enters an f-subshell.
• Lanthanoids: 4f series; actinoids: 5f series.
• Most common lanthanoid oxidation state: +3.
• All actinoids are radioactive.
• Lanthanoid contraction is mainly due to poor shielding by 4f electrons.
• Zr and Hf have very similar radii.
• Eu²⁺ and Gd³⁺: 4f⁷.
• Ce⁴⁺: 4f⁰; Yb²⁺: 4f¹⁴.
• Unpaired electrons generally produce paramagnetism.
• Always distinguish atomic number, electronic configuration and oxidation state.

9. Practice Questions for Class 12 & JEE Main

Attempt these questions before opening the answers. Each question has one correct option.

Q1. The differentiating electron of f-block elements enters which subshell?

A. s-subshell
B. p-subshell
C. d-subshell
D. f-subshell

Show answer and explanation

Answer: D. f-subshell. The defining feature of the f-block is the entry of the differentiating electron into an f-subshell.

Q2. Which electronic configuration is associated with Eu²⁺?

A. 4f⁶
B. 4f⁷
C. 4f⁸
D. 4f¹⁴

Show answer and explanation

Answer: B. 4f⁷. Europium commonly forms Eu²⁺ with a half-filled 4f subshell.

Q3. What is the principal cause of lanthanoid contraction?

A. Strong shielding by 4f electrons
B. Poor shielding by 4f electrons
C. Decreasing nuclear charge
D. Increasing number of shells

Show answer and explanation

Answer: B. Poor shielding by 4f electrons. Increasing nuclear charge is not fully screened by the added 4f electrons, so the effective nuclear attraction increases.

Q4. Which pair has very similar atomic and ionic radii because of lanthanoid contraction?

A. Na and K
B. Li and Cs
C. Zr and Hf
D. Mg and Ba

Show answer and explanation

Answer: C. Zr and Hf. Lanthanoid contraction makes the size of Hf similar to that of Zr.

Q5. Which statement about actinoids is correct?

A. All actinoids are radioactive
B. All actinoids show only +3 oxidation state
C. All actinoids have completely filled 5f subshells
D. All actinoids are stable in nature

Show answer and explanation

Answer: A. All actinoids are radioactive. Radioactivity is a characteristic of every actinoid.

Q6. Which ion has a 4f¹⁴ configuration?

A. Ce⁴⁺
B. Eu²⁺
C. Gd³⁺
D. Yb²⁺

Show answer and explanation

Answer: D. Yb²⁺. Ytterbium loses its two 6s electrons to form Yb²⁺, leaving a filled 4f subshell.

Q7. Which lanthanoid ion is expected to be diamagnetic because it has no unpaired 4f electrons?

A. Gd³⁺ (4f⁷)
B. La³⁺ (4f⁰)
C. Nd³⁺ (4f³)
D. Dy³⁺ (4f⁹)

Show answer and explanation

Answer: B. La³⁺. Its 4f subshell is empty, so all its electrons are paired. The other listed ions have unpaired 4f electrons.

Q8. Which oxidation state is most common for lanthanoids?

A. +1
B. +2
C. +3
D. +6

Show answer and explanation

Answer: C. +3. The +3 state is the characteristic and most common oxidation state for lanthanoids, although exceptions occur.

Q9. Which ion has a half-filled 4f subshell?

A. Ce⁴⁺
B. Gd³⁺
C. La³⁺
D. Lu³⁺

Show answer and explanation

Answer: B. Gd³⁺. Gadolinium(III) has the configuration 4f⁷, a half-filled 4f subshell.

Q10. Why are lanthanoids difficult to separate from one another?

A. They all have different charge numbers
B. They have highly dissimilar chemical properties
C. They have similar ionic radii and chemical properties
D. They all have completely filled 4f subshells

Show answer and explanation

Answer: C. Similar ionic radii and chemical properties. Most lanthanoids form +3 ions with closely related sizes and chemistry, making separation difficult.

🎯 Score yourself

8–10 correct: Excellent revision!

5–7 correct: Revise the trends and practise again.

0–4 correct: Review the notes before attempting another set.

10. Frequently Asked Questions

Are f-block elements included in Class 12 Chemistry?

They are traditionally taught in senior-secondary inorganic chemistry. Always check the current CBSE syllabus and the official JEE Main syllabus for the examination year.

What is the most important topic for JEE Main?

Prioritise electronic configurations, oxidation states, lanthanoid contraction, magnetic properties and comparisons between lanthanoids and actinoids.

Why is the +3 oxidation state common among lanthanoids?

Loss of two 6s electrons and one additional electron commonly produces stable Ln³⁺ ions across the series.

Are all f-block elements radioactive?

No. All actinoids are radioactive, but not all lanthanoids are radioactive. Promethium has no stable isotopes.

Final Revision Reminder

Understand the reason behind every trend instead of memorising isolated f