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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 of the subsequent 4d and 5d transition‑series elements.

hard 5 marks Class 12 · Chemistry · d- and f-Block Elements
Answer
Lanthanide contraction is caused by the poor shielding of the nuclear charge by the 4f electrons, which leads to a steady increase in effective nuclear charge and a gradual decrease in ionic radii from to ; this reduced size makes the 4d elements smaller and more electropositive, bringing their chemistry closer to that of the 5d series, influencing oxidation‑state stability and complex‑formation behaviour.

Step-by-step solution

  1. State the definition: Lanthanide contraction refers to the progressive decrease in atomic and ionic radii observed from lanthanum to lutetium.
  2. Explain the cause: 4f electrons are inefficient at shielding the increasing nuclear charge; each added proton is felt more strongly by the outer electrons, pulling them closer.
  3. Show the result on ionic radii: Plotting values shows a smooth decline (~0.1 Å) across the series, despite the addition of electrons to the inner 4f shell.
  4. Describe the impact on the 4d series: Because the 4d elements (Y, Zr, Nb, etc.) now experience a higher effective nuclear charge, their atomic radii become comparable to the 5d elements, leading to higher ionisation energies and greater tendency to exhibit higher oxidation states similar to 5d metals.
  5. Mention consequences for chemical behaviour: Bond lengths in compounds of 4d metals are shortened, complexes become more stable, and the separation between 4d and 5d trends diminishes, affecting catalytic and magnetic properties.
  6. Summarise the three points to complete the explanation.
Common mistake: Assuming the 4f electrons shield as effectively as d electrons, or believing the contraction stops after Gd rather than continuing to Lu

Concept tested: Lanthanide contraction and its periodic‑trend consequences

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