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

Concept: Lanthanoids (elements from La to Lu) involve filling of the 4f orbitals. These electrons do not shield nuclear charge effectively, resulting in a steady increase in effective nuclear attraction across the series.
Definition of Lanthanoid Contraction: Lanthanoid contraction refers to the gradual decrease in atomic and ionic sizes of lanthanoid elements from lanthanum (La) to lutetium (Lu) with increasing atomic number.
Reason:
Addition of electrons occurs in the 4f orbitals.
4f electrons have poor shielding effect.
Effective nuclear charge increases across the series.
This pulls electrons closer to the nucleus, reducing size.

Consequences of Lanthanoid Contraction:
Similarity Between 4d and 5d Elements: Elements of second and third transition series (e.g., Zr and Hf) have nearly identical atomic radii and similar properties.
Decrease in Basic Strength of Hydroxides: Basicity of lanthanoid hydroxides decreases from La(OH)\(_3\) to Lu(OH)\(_3\) due to decreasing ionic size.
Difficulty in Separation of Lanthanoids: Similar sizes and chemical properties make their separation challenging.
Effect on Periodic Trends: Influences atomic size and properties of elements following lanthanoids in the periodic table.