Lanthanide Contraction

Lanthanide Contraction

The atomic size or the ionic radii of tri-positive lanthanide ions decrease steadily from La to Lu due to increasing nuclear charge and electrons entering the inner (n-2) f orbital. This gradual decrease in size with an increasing atomic number is called lanthanide contraction.

1. Introduction

Lanthanide contraction is a unique phenomenon observed in the lanthanide series of the periodic table. It refers to the gradual decrease in the atomic and ionic radii of lanthanide elements (from Lanthanum (La) to Lutetium (Lu)) as the atomic number increases.

2. Understanding Lanthanides

• Lanthanides are the 15 elements with atomic numbers from 57 (Lanthanum) to 71 (Lutetium).
• They belong to the f-block of the periodic table and are also known as rare earth elements.
• They have a general electronic configuration: $$[Xe] 4f^n 6s^2$$ (where $n = 0$ to $14$)

3. Cause of Lanthanide Contraction

The primary reason for lanthanide contraction is poor shielding effect of 4f orbitals.

• In each lanthanide, an extra proton is added to the nucleus, increasing the nuclear charge.
• Simultaneously, an additional electron is added to the 4f orbital.
• However, 4f electrons do not shield the increasing nuclear charge effectively.
• As a result, the increasing attraction between the nucleus and outer electrons pulls them closer, reducing the atomic and ionic radii.

This gradual shrinkage in atomic and ionic size across the lanthanide series is called lanthanide contraction.

4. Trends in Atomic and Ionic Radii

• The atomic radii of lanthanides decrease slightly from La (1.87 Å) to Lu (1.72 Å).
• The tripositive ionic radii ($M^{3+}$) decrease significantly from La³⁺ (1.06 Å) to Lu³⁺ (0.85 Å).

5. Consequences of Lanthanide Contraction

Lanthanide contraction has several important effects:

1. Similarities in Chemistry of 2nd and 3rd Transition Series

   • Due to lanthanide contraction, elements of the 2nd (Yttrium, Zirconium, etc.) and 3rd transition series (Hafnium, Tantalum, etc.) have nearly the same size.
   • Example: Zr (160 pm) and Hf (159 pm) have nearly the same atomic radius, making their chemical properties similar (Zirconium-Hafnium similarity).

2. Increased Density of Later Lanthanides

   • Due to smaller atomic radii but increasing atomic mass, later lanthanides (like Lutetium) have higher densities.

3. Variation in Basicity of Lanthanides

   • Larger lanthanides (La³⁺, Ce³⁺) form more basic oxides and hydroxides.
   • Smaller lanthanides (Lu³⁺) form less basic compounds due to higher charge density.

4. Poor Separation of Lanthanides

   • Since all lanthanides have similar sizes and charge (+3), their chemical properties are very similar, making it difficult to separate them from each other.

6. Conclusion

Lanthanide contraction is a gradual decrease in atomic and ionic radii across the lanthanide series due to the poor shielding effect of 4f electrons. It influences the chemistry of lanthanides, transition elements, and their applications in industries like electronics, metallurgy, and medicine.