Lanthanoids: Describe the cause and consequences of Lanthanoid Contraction

Chapter: The d- and f-Block Elements

Topic: Lanthanoids – Lanthanoid Contraction

What is Lanthanoid Contraction?

Lanthanoid contraction is the gradual decrease in the atomic and ionic radii of the lanthanoids with an increase in atomic number from La to Lu.

La → Ce → Pr → Nd → … → Yb → Lu
Atomic / Ionic Size ↓ gradually

Lanthanoid Contraction = Gradual decrease in size across the lanthanoid series

Main Cause

The main cause of lanthanoid contraction is the poor shielding effect of 4f electrons.

As we move from one lanthanoid to the next, the nuclear charge increases because the number of protons increases. At the same time, electrons are progressively added to the 4f subshell.

Nuclear Charge ↑ + Poor 4f Shielding
↓
Effective Nuclear Charge ↑
↓
Atomic / Ionic Radius ↓

The 4f electrons are relatively ineffective in shielding the outer electrons from the increasing nuclear charge. Therefore, the effective nuclear charge experienced by the outer electrons increases.

Poor shielding by 4f electrons is the principal cause of lanthanoid contraction.

Step 1

Atomic number increases.

Protons increase.

Step 2

Electrons enter the 4f subshell.

Step 3

4f electrons shield poorly.

Step 4

Effective nuclear charge increases.

Z_eff ↑ → Attraction of nucleus ↑ → Radius ↓

Thus, despite the addition of electrons, the atomic and ionic sizes decrease gradually across the series.

The size of lanthanoid atoms and especially their M³⁺ ions decreases progressively from La to Lu.

Element	Atomic Number	Trend in M³⁺ size
La	57	Largest
Ce	58	↓
Nd	60	↓
Gd	64	↓
Lu	71	Smallest

La³⁺ > Ce³⁺ > Pr³⁺ > … > Lu³⁺

Major Consequences

1. Similarity between 4d and 5d Transition Elements

Because of lanthanoid contraction, the atomic radii of the elements following the lanthanoids in the 5d series become very similar to those of the corresponding 4d elements.

Zr and Hf have nearly similar atomic radii

Therefore, Zr and Hf show very similar chemical properties and are difficult to separate.

2. Difficulty in Separation of Lanthanoids

The lanthanoids have very similar ionic radii and generally exhibit the +3 oxidation state. Hence, their chemical properties are very similar, making their separation difficult.

3. Basicity of Lanthanoid Hydroxides

The basicity of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃.

La(OH)₃ > ... > Lu(OH)₃
Basicity decreases

As the size of the Ln³⁺ ion decreases, its charge density increases and the hydroxide ion is held more strongly.

4. Similarity of 4d and 5d Series

Lanthanoid contraction causes the radii of the second and third transition series to become very similar.

The most important consequence is the close similarity between Zr and Hf.

Normally, atomic size increases when we move from the 4d to the 5d series. However, the expected increase is largely cancelled by lanthanoid contraction.

Expected size increase
−
Lanthanoid contraction
↓
Nearly same size

Therefore, zirconium (Zr) and hafnium (Hf) have almost identical atomic and ionic radii.

Zr and Hf → Nearly identical radii → Very similar chemical properties

The basicity of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃.

La(OH)₃ > Ce(OH)₃ > ... > Lu(OH)₃

As the Ln³⁺ ion becomes smaller, its polarising power increases. The metal–oxygen bond becomes more covalent and the hydroxide ion becomes less readily available as OH⁻. Therefore, basicity decreases.

Q. Define lanthanoid contraction and state its main cause.

Answer:

Lanthanoid contraction is the gradual decrease in atomic and ionic radii of lanthanoids from La to Lu.

Its main cause is the poor shielding effect of 4f electrons, which causes an increase in effective nuclear charge.

Q. Explain the cause of lanthanoid contraction.

As the atomic number increases across the lanthanoid series, electrons are added to the 4f subshell.

The 4f electrons have poor shielding ability. Therefore, the increase in nuclear charge is not effectively shielded.

Consequently, effective nuclear charge increases and the attraction between the nucleus and electrons becomes stronger.

Effective nuclear charge ↑ → Atomic/Ionic radius ↓

Q. What is lanthanoid contraction? Explain its cause and one important consequence.

Definition

Lanthanoid contraction is the progressive decrease in the atomic and ionic radii of lanthanoids from La to Lu.

Cause

The added 4f electrons shield the nuclear charge poorly. Therefore, effective nuclear charge increases as atomic number increases.

Consequence

The most important consequence is the similarity in size and properties of the 4d and 5d transition elements, especially Zr and Hf.

Poor 4f shielding → Z_eff ↑ → Radius ↓ → Lanthanoid contraction

Q. Describe the cause and consequences of lanthanoid contraction.

Cause

1. Atomic number increases from La to Lu.
2. Electrons are progressively added to the 4f subshell.
3. 4f electrons have poor shielding ability.
4. Effective nuclear charge therefore increases.
5. The attraction between the nucleus and electrons increases.
6. Atomic and ionic radii decrease gradually.

Consequences

1. Lanthanoids have very similar chemical properties.
2. Separation of lanthanoids becomes difficult.
3. Basicity of lanthanoid hydroxides decreases from La to Lu.
4. Zr and Hf have almost identical sizes.
5. 4d and 5d transition elements show considerable similarity.

Lanthanoid Contraction is mainly responsible for the close similarity between Zr and Hf.

Q. Explain lanthanoid contraction in detail. Discuss its cause and important consequences.

1. Definition

Lanthanoid contraction is the gradual decrease in atomic and ionic radii of lanthanoids with increasing atomic number from La to Lu.

La → Lu
Atomic Number ↑
Atomic/Ionic Radius ↓

2. Cause

As we move across the lanthanoid series, electrons are progressively added to the 4f subshell.

The 4f electrons shield the nuclear charge poorly. Thus, the increase in nuclear charge is not completely compensated by the shielding effect.

Poor 4f Shielding
↓
Effective Nuclear Charge ↑
↓
Attraction between Nucleus and Electrons ↑
↓
Atomic/Ionic Radius ↓

3. Consequence – Similarity of Lanthanoids

Since their ionic radii are very similar, lanthanoids show very similar chemical properties. This makes their separation difficult.

4. Consequence – Basicity

The basicity of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃.

La(OH)₃ > ... > Lu(OH)₃

5. Consequence – Zr and Hf

Lanthanoid contraction causes Hf to have nearly the same atomic and ionic radii as Zr, despite Hf belonging to the next transition series. Consequently, Zr and Hf possess very similar chemical properties.

6. Overall Summary

Poor shielding of 4f electrons → Increase in effective nuclear charge → Decrease in atomic/ionic size → Lanthanoid contraction → Similarity of lanthanoids, decreasing basicity and similarity of Zr–Hf

1. Lanthanoid contraction refers to:

A) Increase in atomic mass

B) Gradual decrease in atomic and ionic radii

C) Increase in electronegativity only

D) Decrease in atomic number

✅ Answer

B

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2. Lanthanoid contraction occurs from:

A) La to Lu

B) Lu to La

C) Sc to Zn

D) Ti to Cu

✅ Answer

A) La to Lu

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3. The main cause of lanthanoid contraction is:

A) Strong shielding by 4f electrons

B) Poor shielding by 4f electrons

C) Loss of 5d electrons

D) Loss of 6s electrons

✅ Answer

B

---

4. Electrons are progressively added to which subshell in lanthanoids?

A) 3d

B) 4d

C) 4f

D) 5f

✅ Answer

C) 4f

---

5. The shielding ability of 4f electrons is:

A) Very strong

B) Poor

C) Complete

D) Infinite

✅ Answer

B) Poor

---

6. During lanthanoid contraction, effective nuclear charge:

A) Decreases

B) Increases

C) Remains zero

D) Remains unchanged

✅ Answer

B

---

7. Which ion shows a gradual decrease in size across the lanthanoid series?

A) Ln³⁺

B) H⁺

C) Na⁺

D) Cl⁻

✅ Answer

A) Ln³⁺

---

8. The size of Ln³⁺ ions generally decreases from:

A) Lu³⁺ to La³⁺

B) La³⁺ to Lu³⁺

C) Ce³⁺ to La³⁺

D) Lu³⁺ to Ce³⁺

✅ Answer

B

---

9. Lanthanoid contraction makes which pair very similar in size?

A) Na and K

B) Li and Na

C) Zr and Hf

D) Mg and Ca

✅ Answer

C) Zr and Hf

---

10. Zr and Hf have similar radii mainly because of:

A) Diagonal relationship

B) Lanthanoid contraction

C) Inert pair effect

D) Hydrogen bonding

✅ Answer

B

---

11. The basicity of lanthanoid hydroxides from La to Lu:

A) Increases

B) Decreases

C) Remains constant

D) First increases then decreases

✅ Answer

B) Decreases

---

12. Which hydroxide is more basic?

A) La(OH)₃

B) Lu(OH)₃

C) Both are equal

D) None

✅ Answer

A) La(OH)₃

---

13. Lanthanoids are difficult to separate because they have:

A) Very different chemical properties

B) Very similar ionic sizes and properties

C) Different oxidation states only

D) No chemical properties

✅ Answer

B

---

14. Which orbital is responsible for poor shielding in lanthanoids?

A) s

B) p

C) d

D) f

✅ Answer

D) f

---

15. The effective nuclear charge increases because:

A) Shielding completely increases

B) Nuclear charge increases more than shielding

C) Nuclear charge decreases

D) Electrons disappear

✅ Answer

B

---

16. Which transition series is strongly affected by lanthanoid contraction?

A) 1st transition series

B) 2nd and 3rd transition series

C) Only 1st series

D) No transition series

✅ Answer

B

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17. Lanthanoid contraction results in:

A) Large increase in ionic radius

B) Gradual decrease in ionic radius

C) No change in size

D) Complete loss of electrons

✅ Answer

B

---

18. Which statement is correct?

A) 4f electrons shield very effectively

B) 4f electrons shield poorly

C) 4f electrons are absent

D) Lanthanoids have no f electrons

✅ Answer

B

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19. The major effect of increasing Z across lanthanoids is:

A) Decrease in effective nuclear charge

B) Increase in effective nuclear charge

C) No nuclear change

D) Decrease in proton number

✅ Answer

B

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20. Which is the best summary of lanthanoid contraction?

A) Poor 4f shielding → Z_eff ↑ → Radius ↓

B) Good 4f shielding → Z_eff ↓ → Radius ↑

C) Nuclear charge ↓ → Radius ↓

D) 4f electrons disappear → Radius ↑

✅ Answer

A

🎯 Quick Revision

Definition: La → Lu में atomic और ionic radii का gradual decrease
Main Cause: Poor shielding effect of 4f electrons
Effect: Z_eff ↑ → Size ↓
Consequence 1: Lanthanoids have similar chemical properties
Consequence 2: Separation of lanthanoids is difficult
Consequence 3: Basicity of hydroxides decreases La → Lu
Consequence 4: Zr and Hf have nearly similar size and properties