Question

What is Lanthanoid Contraction? State its consequences.

Hint:

Poor shielding by 4f electrons is the key reason behind lanthanoid contraction and its wide impact on periodic properties.

Answer 1

Step 1: Definition of Lanthanoid Contraction.
Lanthanoid contraction refers to the gradual decrease in the atomic and ionic radii of the lanthanoid elements (from La to Lu) with increasing atomic number. Although electrons are being added to the 4f subshell, the size of the atoms decreases instead of increasing.

This happens because the 4f electrons have poor shielding effect. As the nuclear charge increases across the series, the ineffective shielding by 4f electrons allows the nucleus to pull the outer electrons closer, resulting in a steady decrease in size.

Step 2: Cause of Lanthanoid Contraction.
• Increase in nuclear charge from La (Z = 57) to Lu (Z = 71).
• Poor shielding effect of 4f electrons.
• Greater attraction between nucleus and outer electrons.

Step 3: Consequences of Lanthanoid Contraction.
1. Similarity in Size of 4d and 5d Elements:
Due to lanthanoid contraction, the atomic radii of elements in the 5d series become nearly equal to those of the corresponding 4d series elements. For example, Zr and Hf have almost the same atomic size and similar chemical properties.

2. Difficulty in Separation of Lanthanoids:
Since the decrease in size is very gradual, all lanthanoids have very similar chemical properties. This makes their separation from one another difficult.

3. Increase in Basic Strength Differences:
The basic strength of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃ due to the decrease in ionic size.

4. Effect on Density and Melting Point:
There is a gradual increase in density and some variation in melting points across the series because of decreasing atomic size and increasing atomic mass.

Conclusion.
Thus, lanthanoid contraction is the steady decrease in atomic and ionic radii across the lanthanoid series due to poor shielding of 4f electrons, and it significantly influences the properties of both lanthanoids and other elements in the periodic table.