What is the primary cause of Lanthanoid Contraction?
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Correct Answer
Poor shielding effect of the 4f-electrons
Detailed Explanation
Lanthanoid contraction is the gradual decrease in atomic and ionic radii of the lanthanoid elements from La to Lu with increasing atomic number.
The main reason is the poor shielding ability of the 4f-electrons.
As we move from La to Lu:
- The nuclear charge increases because the atomic number increases.
- Electrons are progressively added to the 4f subshell.
- The 4f-electrons shield the nuclear charge ineffectively.
- Therefore, the effective nuclear charge experienced by the outer electrons increases.
- The electron cloud is pulled closer to the nucleus.
- Hence, atomic and ionic radii gradually decrease.
Increasing nuclear charge + poor 4f shielding → increasing Zeff → decreasing radius
Why is 4f Shielding Poor?
The shielding effectiveness generally follows:
s > p > d > f
Since 4f orbitals have poor penetration and are relatively diffuse, their shielding of the nuclear charge is weak.
Therefore, the increasing nuclear charge is not effectively cancelled by the added 4f-electrons.
Major Consequences of Lanthanoid Contraction
1. Similarity between 4d and 5d elements
For example, Zr and Hf have very similar atomic radii and chemical properties.
2. Decrease in basicity of lanthanoid hydroxides
Basicity generally decreases from La(OH)3 to Lu(OH)3.
3. Difficulty in separation of lanthanoids
Their chemical properties become increasingly similar.
For example, Zr and Hf have very similar atomic radii and chemical properties.
2. Decrease in basicity of lanthanoid hydroxides
Basicity generally decreases from La(OH)3 to Lu(OH)3.
3. Difficulty in separation of lanthanoids
Their chemical properties become increasingly similar.
JEE / NEET Quick Trick
Lanthanoid Contraction = Poor shielding by 4f-electrons
4f shielding ↓ → Zeff ↑ → Radius ↓
4f shielding ↓ → Zeff ↑ → Radius ↓
30 Related MCQs with Solutions
1. The primary cause of lanthanoid contraction is:
A. Increase in atomic mass
B. Poor shielding by 4f-electrons
C. Increase in number of shells
D. Strong shielding by 4f-electrons
Answer: B. Poor shielding by 4f-electrons
2. Lanthanoid contraction refers to:
A. Increase in atomic radius across lanthanoids
B. Decrease in atomic/ionic radii across lanthanoids
C. Increase in ionic charge
D. Decrease in atomic number
Answer: B
3. Which electrons are mainly responsible for lanthanoid contraction?
A. 2p
B. 3d
C. 4f
D. 5d
Answer: C. 4f
4. The shielding efficiency of 4f-electrons is:
A. Very high
B. Moderate
C. Poor
D. Perfect
Answer: C. Poor
5. Across the lanthanoid series, effective nuclear charge generally:
A. Decreases
B. Remains constant
C. Increases
D. Becomes zero
Answer: C. Increases
6. The gradual decrease in ionic radii from La3+ to Lu3+ is called:
A. Inert pair effect
B. Lanthanoid contraction
C. Diagonal relationship
D. Shielding effect
Answer: B
7. Which orbital has the poorest shielding ability?
A. s
B. p
C. d
D. f
Answer: D. f
8. Correct order of shielding effectiveness is:
A. f > d > p > s
B. s > p > d > f
C. p > s > f > d
D. d > f > s > p
Answer: B. s > p > d > f
9. Due to lanthanoid contraction, the size of Ln3+ ions:
A. Increases
B. Decreases
C. Remains unchanged
D. First increases then decreases
Answer: B. Decreases
10. Lanthanoid contraction results mainly from increasing:
A. Atomic volume
B. Effective nuclear charge
C. Shielding effect
D. Number of shells
Answer: B. Effective nuclear charge
11. Which pair has unusually similar atomic radii because of lanthanoid contraction?
A. Na and K
B. Zr and Hf
C. Li and Na
D. Mg and Ca
Answer: B. Zr and Hf
12. The 4f-electrons are added progressively in:
A. Alkali metals
B. Lanthanoids
C. Halogens
D. Noble gases
Answer: B. Lanthanoids
13. Lanthanoid contraction causes the basicity of Ln(OH)3 to generally:
A. Increase across the series
B. Decrease across the series
C. Remain constant
D. Become zero
Answer: B. Decrease
14. Which statement is correct?
A. 4f-electrons shield very effectively
B. 4f-electrons shield poorly
C. 4f-electrons are absent in lanthanoids
D. 4f-electrons increase atomic radius
Answer: B
15. The effective nuclear charge increases because:
A. Nuclear charge increases faster than shielding
B. Nuclear charge decreases
C. 4f shielding is perfect
D. Electrons disappear
Answer: A
16. Which series shows lanthanoid contraction?
A. Li to Ne
B. Sc to Zn
C. La to Lu
D. Na to Ar
Answer: C. La to Lu
17. Which of the following is NOT a consequence of lanthanoid contraction?
A. Similarity of Zr and Hf
B. Decrease in basicity of hydroxides
C. Gradual decrease in Ln3+ radii
D. Large increase in atomic radius from La to Lu
Answer: D
18. Poor shielding is mainly due to the nature of:
A. s-orbitals
B. p-orbitals
C. d-orbitals
D. f-orbitals
Answer: D. f-orbitals
19. As atomic number increases across lanthanoids, nuclear charge:
A. Decreases
B. Increases
C. Remains constant
D. Becomes negative
Answer: B. Increases
20. Lanthanoid contraction is most directly associated with:
A. 4f filling
B. 3d filling
C. 5p filling
D. 6s filling
Answer: A. 4f filling
21. Which statement best explains lanthanoid contraction?
A. Added 4f electrons completely cancel added nuclear charge
B. Added 4f electrons poorly shield the increasing nuclear charge
C. Nuclear charge decreases
D. Number of shells decreases
Answer: B
22. The radius of Hf is unexpectedly similar to that of:
A. Ti
B. Zr
C. Sc
D. Y
Answer: B. Zr
23. Which electrons have the least penetrating power among s, p, d and f?
A. s
B. p
C. d
D. f
Answer: D. f
24. Lanthanoid contraction is a result of competition between:
A. Nuclear charge and shielding
B. Mass and volume
C. Ionization and electron affinity only
D. Melting point and boiling point
Answer: A
25. Which ion has a smaller radius: La3+ or Lu3+?
A. La3+
B. Lu3+
C. Both are equal
D. Cannot be predicted
Answer: B. Lu3+
26. Which property generally decreases from La(OH)3 to Lu(OH)3?
A. Basicity
B. Atomic number
C. Nuclear charge
D. Number of electrons
Answer: A. Basicity
27. Which effect explains the similarity of the second and third transition series?
A. Inert pair effect
B. Lanthanoid contraction
C. Diagonal effect
D. Photoelectric effect
Answer: B. Lanthanoid contraction
28. Which is the correct sequence of shielding effectiveness?
A. f > d > p > s
B. s > p > d > f
C. s > d > p > f
D. p > s > d > f
Answer: B
29. The net effect of lanthanoid contraction is:
A. Increasing size
B. Decreasing size
C. No change in size
D. Random change in size
Answer: B. Decreasing size
30. The best one-line explanation of lanthanoid contraction is:
A. Strong shielding by 4f-electrons
B. Poor shielding by 4f-electrons causing increasing effective nuclear charge
C. Decreasing nuclear charge
D. Addition of new shells
Answer: B
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