Why does the basic strength of lanthanoid hydroxides decrease systematically from $La(OH)_3$ to $Lu(OH)_3$?

Why does the basic strength of lanthanoid hydroxides decrease systematically from La(OH)₃ to Lu(OH)₃? La(OH)₃ से Lu(OH)₃ तक लैन्थेनाइड हाइड्रॉक्साइड्स की क्षारीयता क्रमशः क्यों घटती है? +

Correct Answer

Due to lanthanoid contraction

4f-electrons show poor shielding → effective nuclear charge increases → Ln³⁺ size decreases → hydroxide becomes less ionic → basic strength decreases.

Detailed Explanation

As we move from La to Lu, the atomic number increases and electrons are progressively added to the 4f-subshell. The 4f-electrons have a poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. This causes a gradual decrease in the size of Ln³⁺ ions. This phenomenon is called lanthanoid contraction.

La³⁺   →   Ce³⁺   →   ...   →   Lu³⁺
Size ↓

As Ln³⁺ becomes smaller, its charge density increases and its attraction for the O²⁻ part of OH⁻ becomes stronger. Therefore, the Ln–OH bond develops greater covalent character and the hydroxide becomes less ionic. Since the basic character of metal hydroxides is associated with their ionic character and ease of providing OH⁻, the basic strength decreases.

La(OH)₃ > Ce(OH)₃ > ... > Lu(OH)₃

The Complete Chain

4f-electrons added
↓
Poor shielding
↓
Z_eff increases
↓
Ln³⁺ ionic radius decreases
↓
Charge density increases
↓
Covalent character of Ln–OH increases
↓
Ionic character decreases
↓
Basic strength decreases

Important Comparison

La(OH)₃ has a relatively large La³⁺ ion and greater ionic character, so it is the most basic hydroxide in the series.

Lu(OH)₃ has the smallest Ln³⁺ ion, greater charge density and greater covalent character, so it is the least basic.

Basicity: La(OH)₃ > Lu(OH)₃

JEE Level Quick Trick

Lanthanoid contraction ↑ → Ln³⁺ size ↓ → covalent character ↑ → basicity ↓

30 Related MCQs with Solutions

1. The basic strength of lanthanoid hydroxides from La to Lu generally:

A. Increases

B. Decreases

C. Remains constant

D. First increases then decreases

Answer: B. Decreases

2. The main reason for the decrease in basicity is:

A. Lanthanoid contraction

B. Inert pair effect

C. Diagonal relationship

D. Hydrogen bonding

Answer: A. Lanthanoid contraction

3. Which hydroxide is generally the most basic?

A. La(OH)₃

B. Gd(OH)₃

C. Lu(OH)₃

D. Yb(OH)₃

Answer: A. La(OH)₃

4. Which hydroxide is least basic in the lanthanoid series?

A. La(OH)₃

B. Ce(OH)₃

C. Lu(OH)₃

D. Pr(OH)₃

Answer: C. Lu(OH)₃

5. Lanthanoid contraction is mainly due to poor shielding by:

A. 3d-electrons

B. 4f-electrons

C. 5d-electrons

D. 6p-electrons

Answer: B. 4f-electrons

6. From La³⁺ to Lu³⁺, ionic radius:

A. Increases

B. Decreases

C. Remains constant

D. Becomes zero

Answer: B. Decreases

7. Smaller Ln³⁺ ions have:

A. Lower charge density

B. Higher charge density

C. No charge density

D. Constant charge density

Answer: B. Higher charge density

8. Smaller Ln³⁺ ions tend to form bonds with greater:

A. Ionic character

B. Covalent character

C. Metallic character

D. Hydrogen character

Answer: B. Covalent character

9. Which has greater ionic character?

A. La(OH)₃

B. Lu(OH)₃

C. Both equal

D. Neither

Answer: A. La(OH)₃

10. Basicity of a metal hydroxide generally increases with:

A. Increasing covalent character

B. Increasing ionic character

C. Increasing polarization

D. Decreasing electropositivity

Answer: B. Increasing ionic character

11. Correct order of basicity is:

A. La(OH)₃ < Lu(OH)₃

B. La(OH)₃ > Lu(OH)₃

C. La(OH)₃ = Lu(OH)₃

D. Cannot be predicted

Answer: B

12. The effective nuclear charge across lanthanoids generally:

A. Decreases

B. Increases

C. Remains constant

D. Becomes negative

Answer: B. Increases

13. Which electron type has poor shielding ability?

A. s

B. p

C. d

D. f

Answer: D. f

14. The basicity trend is directly related to:

A. Lanthanoid contraction

B. Atomic mass only

C. Melting point only

D. Neutron number only

Answer: A. Lanthanoid contraction

15. Which ion has the greatest charge density?

A. La³⁺

B. Lu³⁺

C. Both equal

D. La²⁺

Answer: B. Lu³⁺

16. As ionic radius decreases, polarization of OH⁻:

A. Decreases

B. Increases

C. Remains unchanged

D. Becomes zero

Answer: B. Increases

17. Increased polarization generally leads to:

A. Greater covalent character

B. Greater ionic character

C. No bonding

D. Metallic bonding only

Answer: A. Greater covalent character

18. Which factor does NOT primarily control the basicity trend?

A. Ionic size

B. Charge density

C. Covalent character

D. Colour of the hydroxide

Answer: D

19. The smallest Ln³⁺ ion among the following is:

A. La³⁺

B. Ce³⁺

C. Gd³⁺

D. Lu³⁺

Answer: D. Lu³⁺

20. Which statement explains the basicity trend most accurately?

A. Ionic size increases from La to Lu

B. Ionic size decreases and covalent character increases

C. Nuclear charge decreases

D. 4f shielding becomes stronger

Answer: B

21. Lanthanoid contraction results in an increase in:

A. Ionic radius

B. Charge density

C. Atomic volume

D. Ionic character

Answer: B. Charge density

22. Which hydroxide has the highest covalent character?

A. La(OH)₃

B. Ce(OH)₃

C. Lu(OH)₃

D. Pr(OH)₃

Answer: C. Lu(OH)₃

23. Which hydroxide has the lowest covalent character?

A. La(OH)₃

B. Lu(OH)₃

C. Yb(OH)₃

D. Er(OH)₃

Answer: A. La(OH)₃

24. The basicity trend from La to Lu is:

A. La(OH)₃ > Ce(OH)₃ > ... > Lu(OH)₃

B. Lu(OH)₃ > ... > La(OH)₃

C. Constant

D. Random

Answer: A

25. Which phenomenon causes the gradual decrease in Ln³⁺ radius?

A. Lanthanoid contraction

B. Inert pair effect

C. Resonance

D. Hydrogen bonding

Answer: A

26. Poor shielding by 4f-electrons causes:

A. Decrease in Z_eff

B. Increase in Z_eff

C. No change in Z_eff

D. Negative Z_eff

Answer: B. Increase in Z_eff

27. Greater charge density of Lu³⁺ causes:

A. Lower polarization

B. Higher polarization of OH⁻

C. No interaction

D. Complete dissociation always

Answer: B

28. Which relation is correct?

A. Ionic radius ↑ → covalent character ↑

B. Ionic radius ↓ → covalent character ↑

C. Ionic radius ↓ → charge density ↓

D. Ionic radius ↑ → polarization ↑

Answer: B

29. Which factor makes La(OH)₃ more basic than Lu(OH)₃?

A. La³⁺ is larger and the hydroxide has greater ionic character

B. La³⁺ is smaller

C. La has greater 4f shielding

D. Lu has lower nuclear charge

Answer: A

30. The best one-line explanation for decreasing basicity of lanthanoid hydroxides is:

A. Lanthanoid contraction decreases ionic size and increases covalent character

B. Atomic mass decreases from La to Lu

C. Nuclear charge decreases

D. 4f-electrons shield increasingly well

Answer: A

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