Lanthanoids: The 4f Series

Introduction

Lanthanoids (also called lanthanides or rare earth elements) are the 14 elements in the 4f series - from Cerium (Ce) to Lutetium (Lu). Despite their name, they’re not actually rare; they’re called “rare” because they’re difficult to separate from each other!

Position in Periodic Table

Lanthanoid Series:

• Elements: Ce (58) to Lu (71) - 14 elements
• Also includes: Lanthanum (La, 57) - often grouped with them
• Collective name: Lanthanoids or Rare Earth Elements
• Period: 6th period
• Block: f-block (4f orbitals being filled)

graph TD
    A[f-Block Elements] --> B[Lanthanoids]
    A --> C[Actinoids]
    B --> B1[4f series: Ce to Lu]
    B --> B2[58 to 71]
    C --> C1[5f series: Th to Lr]
    C --> C2[90 to 103]

In Periodic Table:

• Placed below the main table
• Interrupt the 5d series
• La → 4f fills → Lu → 5d continues (Hf)

Interactive Demo: Visualize Lanthanoid Position

Explore the position of lanthanoids in the periodic table and understand the 4f series placement.

Electronic Configuration

General Configuration

Complete Electronic Configurations:

Anomalous Configurations:

Lanthanoid Contraction

The most important concept in lanthanoid chemistry!

Definition

Lanthanoid contraction is the steady decrease in atomic and ionic radii of lanthanoid elements from La to Lu.

Radii Data

Ln³⁺ Ionic Radii (in pm):

Total decrease: La³⁺ (106 pm) to Lu³⁺ (85 pm) = 21 pm

Cause of Lanthanoid Contraction

graph TD
    A[Adding electrons to 4f orbitals] --> B[Poor shielding by 4f electrons]
    B --> C[Nuclear charge increases]
    C --> D[Outer electrons pulled closer]
    D --> E[Atomic radius decreases]
    E --> F[Lanthanoid Contraction]

    style F fill:#e74c3c

Detailed Explanation:

1. Across the series: Each element has one more proton (nuclear charge +1)
2. Extra electron goes into: 4f orbital
3. 4f electrons provide: Very poor shielding (diffused orbitals)
4. Result: Effective nuclear charge increases
5. Consequence: Outer electrons pulled closer → radius decreases

Why poor shielding?

• 4f orbitals have complex shape
• 4f electrons are buried deep inside
• Less effective at screening nuclear charge

Consequences of Lanthanoid Contraction

1. Similar Chemical Properties

All lanthanoids behave very similarly:

• Same oxidation state (+3 predominant)
• Similar ionic radii
• Difficult to separate from each other

2. Difficult Separation

Lanthanoids are hard to separate because:

• Very similar ionic radii
• Similar chemical behavior
• Require ion-exchange chromatography or fractional crystallization

3. Effect on Post-Lanthanoid Elements

Most Important Consequence:

The 4d and 5d series elements have similar atomic radii!

Why?

• Hf comes after lanthanoids
• Lanthanoid contraction compensates for expected size increase
• Hf and Zr have almost identical radii despite being in different periods!

Result:

• Zr and Hf are very similar chemically
• Nb and Ta are almost identical twins
• Very difficult to separate (e.g., Zr from Hf)

4. Variation in Basic Strength of Hydroxides

Reason:

• Smaller cation → higher charge density
• Stronger M-O bond → less ionization
• Less OH⁻ released → weaker base

5. Trends in Properties

Due to lanthanoid contraction:

• Density increases: La to Lu
• Melting point increases: Generally (irregular)
• Hardness increases: La to Lu
• Ionic character decreases: Of M-X bonds

General Characteristics

1. Physical Properties

Appearance:

• Silvery-white metals
• Soft (can be cut with knife)
• High luster when freshly cut
• Tarnish rapidly in air

Metallic Properties:

• Good electrical conductivity
• Moderate thermal conductivity
• High melting points (except Eu, Yb)
• High density

2. Chemical Properties

Reactivity:

• Quite reactive (similar to Ca)
• Readily form oxides in air: 4M + 3O₂ → 2M₂O₃
• React with water (slowly): 2M + 6H₂O → 2M(OH)₃ + 3H₂
• React with halogens: 2M + 3X₂ → 2MX₃
• React with acids: 2M + 6HCl → 2MCl₃ + 3H₂

Reducing Nature:

• Good reducing agents (especially Eu, Yb)
• Standard reduction potential: E° ≈ -2.3 V

3. Oxidation States

Most Common: +3

The +3 oxidation state is most stable for all lanthanoids.

Other Oxidation States (Less Common):

Stability of +2 and +4:

1. +4 state (Ce⁴⁺):

   • Ce⁴⁺ = [Xe] (noble gas configuration)
   • Very stable, used as oxidizing agent

2. +2 states (Eu²⁺, Yb²⁺):

   • Eu²⁺ = [Xe]4f⁷ (half-filled)
   • Yb²⁺ = [Xe]4f¹⁴ (fully-filled)
   • Both stable

4. Magnetic Properties

All lanthanoid ions (except La³⁺ and Lu³⁺) are paramagnetic due to unpaired f-electrons.

Magnetic Moment:

Not strictly spin-only! Must consider orbital contribution:

where J = total angular momentum quantum number

Paramagnetism:

Gd³⁺ (f⁷) has the highest magnetic moment due to 7 unpaired electrons.

5. Color of Lanthanoid Ions

Many Ln³⁺ ions are colored due to f-f transitions (weak, forbidden transitions).

Colors in Aqueous Solution:

Colorless ions: La³⁺ (f⁰), Gd³⁺ (f⁷, half-filled), Lu³⁺ (f¹⁴)

Why colored?

• f-f transitions in visible region
• Forbidden transitions (weak absorption)
• Colors are pale compared to d-block elements

6. Complex Formation

Lanthanoids have less tendency to form complexes compared to d-block elements.

Reasons:

1. Large ionic size (low charge density)
2. No vacant d-orbitals for bonding
3. +3 charge is moderate

Some complexes:

• [Ce(NO₃)₆]²⁻
• Lanthanoid chelates with EDTA
• Organometallic compounds

Coordination Numbers:

• Usually high: 8, 9, or even 12
• Due to large size

Comparison with Actinoids

*Except Pm-61 (all isotopes radioactive)

Applications of Lanthanoids

Industrial Uses

1. Permanent Magnets:

• Nd-Fe-B magnets (Neodymium): Strongest permanent magnets
• Used in hard drives, speakers, electric motors

2. Catalysts:

• Ce compounds: Catalytic converters in automobiles
• La: Petroleum cracking catalysts

3. Phosphors and LEDs:

• Eu, Tb: Red and green phosphors in LEDs, TVs
• Ce: Scintillation counters

4. Glass and Ceramics:

• Ce oxide: Glass polishing powder
• Pr, Nd: Colored glass and welder’s goggles

5. Lasers:

• Nd:YAG laser (Neodymium-doped yttrium aluminum garnet)

6. Hydrogen Storage:

• LaNi₅: Rechargeable battery electrodes

7. Medical Applications:

• Gd compounds: MRI contrast agents

8. Metallurgy:

• Mischmetal (Ce-rich mixture): Added to steels and alloys

Separation of Lanthanoids

Extremely difficult due to:

1. Very similar chemical properties
2. Nearly identical ionic radii
3. All show +3 oxidation state

Methods

1. Ion Exchange Chromatography:

• Most effective modern method
• Uses ion-exchange resins
• Smaller ions (Lu³⁺) bind more strongly
• Eluted in order: La → Ce → … → Lu

2. Solvent Extraction:

• Uses organic solvents
• Exploits slight differences in partition coefficients

3. Fractional Crystallization:

• Traditional method
• Repeated crystallization of salts
• Very tedious (hundreds of cycles needed!)

Common JEE Mistakes

1. Lanthanum is NOT a lanthanoid

   • Lanthanoids: Ce (58) to Lu (71)
   • Lanthanum (57) is often grouped with them but technically separate
   • La has no f-electrons: [Xe]5d¹6s²

2. Lanthanoid contraction causes

   • Wrong: Filled f-orbitals cause contraction
   • Correct: POOR SHIELDING by f-electrons causes contraction

3. Similar radii of 4d and 5d

   • Wrong: They’re in same group
   • Correct: Lanthanoid contraction compensates for expected increase

4. Color origin

   • d-block: d-d transitions (strong, bright colors)
   • f-block: f-f transitions (weak, pale colors)

5. Oxidation states

   • ALL lanthanoids show +3 as primary state
   • Only Ce (+4), Eu (+2), Yb (+2) are notable exceptions

6. Promethium exception

   • Pm-61 is radioactive (no stable isotopes)
   • All other lanthanoids are NOT radioactive

Practice Problems

Level 1: Basic Concepts

1. What is lanthanoid contraction? Write its causes.

2. Why are lanthanoids difficult to separate from each other?

3. Write electronic configuration of:

   • Ce (Z=58)
   • Gd (Z=64)
   • Lu (Z=71)

4. Which lanthanoid ions are diamagnetic?

Level 2: Application

5. Explain the following:

   • Ce⁴⁺ is a good oxidizing agent
   • Eu²⁺ is a good reducing agent
   • Gd³⁺ is colorless despite having 7 f-electrons

6. Compare:

   • Ionic radii of La³⁺ and Lu³⁺
   • Atomic radii of Zr and Hf
   • Basic strength of La(OH)₃ and Lu(OH)₃

7. Why do lanthanoids show limited oxidation states compared to actinoids?

Level 3: JEE Advanced

8. Arrange in order of decreasing ionic radii: Ce³⁺, Eu³⁺, Gd³⁺, Lu³⁺

9. The most common oxidation state of lanthanoids is:

   • (a) +2
   • (b) +3
   • (c) +4
   • (d) +5

10. Which property is NOT a consequence of lanthanoid contraction?

    • (a) Similar radii of Zr and Hf
    • (b) Difficulty in separation of lanthanoids
    • (c) Decrease in basic character from La(OH)₃ to Lu(OH)₃
    • (d) Radioactivity of lanthanoids

11. Assertion (A): Lanthanoid contraction is caused by poor shielding of 4f electrons. Reason (R): The size of Hf is similar to Zr due to lanthanoid contraction.

    • (a) Both A and R true, R explains A
    • (b) Both true, R doesn’t explain A
    • (c) A true, R false
    • (d) Both false

12. Calculate the total decrease in ionic radius from La³⁺ (106 pm) to Lu³⁺ (85 pm). What is the average decrease per element?

Memory Tricks

“LCPN” for Lanthanoid Series

Starts with:

• Lanthanum (though not technically a lanthanoid)
• Cerium (first true lanthanoid)
• Praseodymium
• Neodymium
• …and so on

Lanthanoid Contraction

“Poor Shielding = Smaller Size”

• 4f electrons shield poorly
• Nuclear charge increases
• Size decreases across series

Variable Oxidation States

“CE uses 4, EU and YB use 2”

• Ce: +4 (noble gas config)
• Eu: +2 (half-filled f⁷)
• Yb: +2 (fully-filled f¹⁴)

Consequences of Contraction

“SZBD” (Size Decreases, But Difficult)

• Separation is difficult
• Zr and Hf similar (also Nb-Ta, Mo-W)
• Basicity decreases (La(OH)₃ > Lu(OH)₃)
• Density increases

Related Topics

Within d-f Block Elements

• Transition Elements - d-block properties
• Properties of d-Block - Comparison with f-block
• Actinoids - 5f series comparison
• Important Compounds - Ce⁴⁺ as oxidizer

Other Chemistry Topics

• Atomic Structure - f-orbital shapes
• Periodic Classification - Periodic trends
• Chemical Bonding - Complex formation

Cross-Subject Connections

• Magnetism - Paramagnetic materials
• Nuclear Chemistry - Promethium radioactivity