Question

What is Lanthanide contraction? What are the consequences of Lanthanide contraction?

Hint:

It is the gradual size decrease across 4f elements due to poor \(4f\) shielding; recall Zr/Hf similarity and falling hydroxide basicity.

Answer 1

Step 1: Definition. Lanthanide contraction is the steady, small decrease in the atomic and ionic radii of the lanthanide elements (from La/Ce to Lu) as the atomic number increases across the series.
Step 2: Cause. As we move along the series, each added electron enters an inner \(4f\) orbital. The \(4f\) electrons shield the outer electrons from the nucleus very poorly (diffuse, poorly directed orbitals). So the effective nuclear charge felt by the outer electrons keeps rising, pulling them inward and shrinking the size.
Step 3: Consequences.
(i) The atomic radii of the elements of the second (4d) and third (5d) transition series become almost the same (e.g. Zr and Hf, Nb and Ta), making these pairs very hard to separate.
(ii) The lanthanides themselves have closely similar sizes, so their chemical properties are very alike and separating them is difficult.
(iii) The basic strength of the hydroxides \(M(OH)_3\) decreases from \(La(OH)_3\) to \(Lu(OH)_3\) as the ionic size falls.
(iv) There is a slight, regular variation in properties such as ionization enthalpy across the series.
Step 4: Summary. Poor \(4f\) shielding causes a gradual size decrease, whose main results are the Zr/Hf-type resemblance, difficult separation and falling basicity.
\[\boxed{\text{Gradual } r \downarrow \text{ due to poor } 4f \text{ shielding}}\]