Question

Explain Lanthanoid Contraction and its consequences on the atomic radii of 5d series elements.

Hint:

Poor shielding by 4f electrons is the key reason behind lanthanoid contraction.

Answer 1

Concept: Lanthanoid contraction refers to the gradual decrease in atomic and ionic radii of lanthanoids with increasing atomic number due to poor shielding of nuclear charge.
Answer:
Lanthanoid Contraction:
It is the steady decrease in the size of lanthanoid elements (from La to Lu) with increasing atomic number. This occurs because the added electrons enter the 4f orbitals, which have poor shielding effect. As a result, the effective nuclear charge increases, pulling electrons closer to the nucleus.
Consequences on 5d Series Elements:

• Similar Atomic Radii: The atomic radii of 5d elements become almost equal to those of corresponding 4d elements (e.g., Zr and Hf have nearly the same size).
• Difficulty in Separation: Elements with similar sizes and properties are difficult to separate.
• Higher Density: 5d elements have higher densities due to increased nuclear charge and compact size.
• Greater Stability of Complexes: Due to smaller size, 5d elements form more stable complexes.

Thus, lanthanoid contraction significantly influences the properties of 5d transition elements.