Lesson eLearning

Explain the phenomenon of lanthanide contraction. In your answer discuss (i) the cause of the contraction, (ii) how it influences the ionic radii across the lanthanide series, and (iii) the effect of this contraction on the chemical properties and periodic trends of the subsequent 4d and 5d transition‑series elements, especially regarding atomic radius, coordination numbers and bonding characteristics.

hard 4 marks Class 12 · Chemistry · d- and f-Block Elements
Answer
Lanthanide contraction is caused by poor shielding of the nuclear charge by the 4f electrons; as the nuclear charge increases across the series, the ineffective shielding results in a gradual decrease in ionic radii. This reduced size influences the 4d and 5d series by making their atoms smaller, increasing effective nuclear charge, leading to higher oxidation states, lower coordination numbers and stronger metal‑ligand bonds.

Step-by-step solution

  1. State that the 4f electrons added across the lanthanide series shield the increased nuclear charge only weakly.
  2. Explain that this weak shielding causes a steady increase in effective nuclear charge felt by the outer electrons.
  3. Show that the higher effective nuclear charge pulls the electron cloud inward, giving a gradual decrease in ionic radii from La³⁺ to Lu³⁺ (lanthanide contraction).
  4. Describe how the smaller radii of lanthanide ions lead to higher charge density and greater polarising power.
  5. Link the contraction to the 4d and 5d series: because the lanthanide contraction precedes them, the 4d and 5d elements have radii similar to their 3d counterparts.
  6. Explain that the reduced size results in higher oxidation states becoming more stable (e.g., +4 for Zr, Nb, Mo, etc.).
  7. Discuss that smaller atomic size limits the number of ligands that can surround the metal centre, often lowering the coordination number (e.g., from 8 to 6).
  8. Conclude that stronger metal‑ligand overlap due to shorter bond lengths enhances bond strength, influencing colour, magnetic behaviour and catalytic activity of 4d/5d complexes.
Common mistake: Attributing the contraction solely to increasing nuclear charge without mentioning poor 4f shielding

Concept tested: Lanthanide contraction and its consequences

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