Question

What is lanthanoid contraction and what are the effects of lanthanoid contraction?

Answer 1

As we move across the 4f series, the atomic and ionic radii of lanthanoids show a gradual decrease with an increase in atomic number. This decrease in ionic size is called lanthanoid contraction.

Effects or Consequences of lanthanoid contraction:

1. Basicity differences: As we move from Ce³⁺ to Lu³⁺, the basic character of Ln³⁺ ions decreases. Due to the decrease in the size of Ln³⁺ ions, the ionic character of the Ln – OH bond decreases (covalent character increases) which results in a decrease in the basicity.

2. Similarities among lanthanoids: In the complete f-series only a 10 pm decrease in atomic radii and a 20 pm decrease in ionic radii are observed. Because of this very small change in radii of lanthanoids, their chemical properties are quite similar.

The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series due to lanthanoid contraction.

For example,

• 4d series – Zr – Atomic radius 145 pm
• 5d series – Hf – Atomic radius 144 pm