Explain the phenomenon of lanthanide contraction, describing (i) the cause of the contraction and (ii) how it influences the ionic radii across the lanthanide series.
Answer
Lanthanide contraction is caused by poor shielding of the nuclear charge by 4f electrons, leading to a progressive decrease in ionic radii; this causes the radii to shrink across the series.
Step-by-step solution
- 1. In lanthanides, the 4f orbitals are added one after another as the atomic number increases.
- 2. The 4f electrons are poorly shielded by the already occupied 4f and 5d orbitals; they do not effectively block the increasing nuclear charge.
- 3. Consequently, the effective nuclear charge experienced by the outer electrons increases progressively from La to Lu.
- 4. With a stronger pull from the nucleus, the electron cloud contracts, decreasing the ionic radius.
- 5. The contraction is evident as the radii of successive lanthanide ions decrease gradually, despite the addition of a new electron shell.
- 6. This trend leads to smaller ionic sizes for late lanthanides (e.g., Lu³⁺) compared to early ones (e.g., La³⁺).
Common mistake: Attributing the contraction to increased nuclear charge alone, ignoring poor shielding by 4f electrons
Concept tested: Lanthanide contraction effect on radii
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