1. Lanthanide contraction is due to poor shielding of which orbital?
+
Correct Answer
4f orbital
Explanation
Lanthanide contraction is the gradual decrease in the atomic and ionic radii
of the lanthanide elements from La to Lu.
The main reason is the poor shielding ability of the 4f electrons.
Increasing nuclear charge + poor 4f shielding
↓
Increasing effective nuclear charge (Zeff)
↓
Decrease in atomic/ionic radius
As we move across the lanthanide series, electrons are progressively added
to the 4f subshell. The 4f electrons do not shield the
increasing nuclear charge effectively.
Therefore, the outer electrons experience a greater effective nuclear
attraction, causing the size to decrease gradually.
↓
Increasing effective nuclear charge (Zeff)
↓
Decrease in atomic/ionic radius
La
Beginning of series
Relatively larger radius
Relatively larger radius
4f filling
Poor shielding
Zeff ↑
Zeff ↑
Lu
End of series
Smaller radius
Smaller radius
La → Lu : Atomic/Ionic radius ↓
JEE-Level Concept
The shielding effectiveness of subshells follows approximately:
s > p > d > f
Thus, 4f electrons are particularly poor at shielding.
Consequently, the increasing nuclear charge across the lanthanide series is
not adequately cancelled by shielding.
This produces a steady increase in effective nuclear charge.
Zeff = Z − σ
Here, Z increases significantly while the shielding
contribution of the newly added 4f electrons is relatively poor.
Hence:
4f poor shielding → Zeff ↑ → radius ↓
Important Consequences of Lanthanide Contraction
1. Similarity of Zr and Hf:
Due to lanthanide contraction, Zr and Hf have very similar atomic and ionic radii and therefore show similar chemical properties.
2. Difficulty in separating lanthanides:
Their ionic radii are very similar, making their chemical properties highly similar.
3. Basicity of lanthanide hydroxides:
Basicity generally decreases from La(OH)3 to Lu(OH)3 because ionic size decreases and covalent character increases.
Due to lanthanide contraction, Zr and Hf have very similar atomic and ionic radii and therefore show similar chemical properties.
2. Difficulty in separating lanthanides:
Their ionic radii are very similar, making their chemical properties highly similar.
3. Basicity of lanthanide hydroxides:
Basicity generally decreases from La(OH)3 to Lu(OH)3 because ionic size decreases and covalent character increases.
Quick JEE Trick
Whenever you see:
“Lanthanide contraction is due to poor shielding of which electrons?”
Immediately remember:
4f electrons
And the complete chain is:
4f poor shielding → Zeff ↑ → radius ↓
30 Related JEE-Level MCQs with Solutions
1. Lanthanide contraction is mainly due to poor shielding by:
A. 3d electrons
B. 4f electrons
C. 5d electrons
D. 6s electrons
Answer: B. 4f electrons
2. Across the lanthanide series, the atomic radius generally:
A. Increases
B. Decreases
C. Remains constant
D. First increases then decreases
Answer: B. Decreases
3. The electrons progressively added across the lanthanide series enter:
A. 3d
B. 4f
C. 5p
D. 6s
Answer: B. 4f
4. Which subshell has the poorest shielding ability?
A. s
B. p
C. d
D. f
Answer: D. f
5. Lanthanide contraction causes effective nuclear charge to:
A. Decrease
B. Increase
C. Become zero
D. Remain unchanged
Answer: B. Increase
6. Which pair has similar atomic radii due to lanthanide contraction?
A. Na and K
B. Zr and Hf
C. Li and Cs
D. F and Cl
Answer: B. Zr and Hf
7. The lanthanide contraction occurs mainly from:
A. La to Lu
B. Na to Ar
C. Sc to Zn
D. K to Kr
Answer: A. La to Lu
8. Which statement best explains lanthanide contraction?
A. Increasing nuclear charge is poorly shielded by 4f electrons
B. Nuclear charge decreases
C. 4f electrons completely shield the nucleus
D. Electron shells are removed
Answer: A
9. Which quantity increases across the lanthanide series?
A. Effective nuclear charge
B. Atomic radius
C. Ionic radius
D. Shielding efficiency of 4f electrons
Answer: A
10. Poor shielding by 4f electrons results in:
A. Expansion of electron cloud
B. Contraction of electron cloud
C. Loss of nuclear charge
D. Increase in shell number
Answer: B
11. Which has better shielding ability?
A. 4f
B. 4d
C. 4p
D. 4s
Answer: D. 4s
12. Approximate shielding order is:
A. f > d > p > s
B. s > p > d > f
C. p > s > f > d
D. d > f > s > p
Answer: B
13. Which element is at the end of the lanthanide series?
A. La
B. Ce
C. Yb
D. Lu
Answer: D. Lu
14. Lanthanide contraction is most directly associated with filling of:
A. 3d orbitals
B. 4f orbitals
C. 5p orbitals
D. 6p orbitals
Answer: B
15. The chemical similarity of Zr and Hf is primarily due to:
A. Identical atomic numbers
B. Lanthanide contraction
C. Same electronic configuration
D. Same mass number
Answer: B
16. Which ion generally shows decreasing radius across lanthanides?
A. Ln3+
B. Na+
C. K+
D. Cl−
Answer: A. Ln3+
17. The major reason for difficult separation of lanthanides is their:
A. Large difference in ionic radii
B. Similar ionic radii and chemical properties
C. Different oxidation states only
D. High volatility
Answer: B
18. Basicity of Ln(OH)3 generally:
A. Increases from La to Lu
B. Decreases from La to Lu
C. Remains exactly constant
D. Becomes zero immediately
Answer: B
19. Lanthanide contraction is a consequence of poor shielding and increasing:
A. Neutron number only
B. Nuclear charge
C. Shell number only
D. Atomic mass only
Answer: B
20. Which electrons are least effective in shielding nuclear charge?
A. s-electrons
B. p-electrons
C. d-electrons
D. f-electrons
Answer: D. f-electrons
21. The contraction across lanthanides is gradual because:
A. Nuclear charge changes randomly
B. 4f electrons are added progressively
C. 6s electrons are continuously removed
D. No electrons are added
Answer: B
22. Which subshell is filled in the lanthanide series?
A. 3d
B. 4f
C. 5d
D. 6p
Answer: B
23. The decrease in Ln3+ radius from La3+ to Lu3+ is called:
A. Diagonal relationship
B. Lanthanide contraction
C. Inert pair effect
D. Relativistic effect
Answer: B
24. Which effect is primarily responsible for the poor shielding of 4f electrons?
A. Strong penetration of 4f electrons
B. Diffuse nature and poor penetration of f orbitals
C. Complete nuclear shielding
D. Absence of electron–electron repulsion
Answer: B
25. Lanthanide contraction influences the size of elements immediately following the lanthanides, especially:
A. 4d transition elements
B. Alkali metals
C. Halogens only
D. Noble gases only
Answer: A
26. Which pair is most famously explained by lanthanide contraction?
A. Na and K
B. Zr and Hf
C. Li and Na
D. F and Cl
Answer: B
27. Effective nuclear charge on outer electrons across lanthanides:
A. Generally increases
B. Generally decreases
C. Remains zero
D. Becomes negative
Answer: A
28. Which is the correct causal sequence?
A. 4f poor shielding → Zeff ↑ → radius ↓
B. 4f good shielding → Zeff ↓ → radius ↓
C. 4f poor shielding → Zeff ↓ → radius ↑
D. 4f electrons removed → radius ↑
Answer: A
29. Which statement is incorrect?
A. 4f electrons shield poorly
B. Effective nuclear charge increases across lanthanides
C. Lanthanide ionic radii generally decrease
D. 4f electrons shield better than s-electrons
Answer: D
30. The one-line answer to the question is:
A. Poor shielding by 3d electrons
B. Poor shielding by 4f electrons
C. Poor shielding by 5d electrons
D. Poor shielding by 6s electrons
Answer: B. Poor shielding by 4f electrons
Advertisement
