Explain the phenomenon of lanthanide contraction and its impact on the periodic trends of the subsequent 4d and 5d transition‑series elements, especially regarding atomic radius, coordination numbers and bonding characteristics.
Answer
Lanthanide contraction occurs because 4f electrons shield poorly, increasing effective nuclear charge and reducing ionic radii, which in turn decreases atomic radius, coordination numbers and bond lengths of succeeding 4d/5d transition metals, making their chemistry more covalent.
Step-by-step solution
- Lanthanide contraction refers to the gradual decrease in ionic radii of the lanthanides from La3+ to Lu3+.
- This contraction is caused by poor shielding of the increasing nuclear charge by the 4f electrons, which are added one by one while their penetration is small.
- Consequently, the effective nuclear charge felt by outer electrons increases, pulling them closer to the nucleus and reducing the size of the ions.
- The smaller ionic sizes of the lanthanides influence the elements that follow, the 4d (e.g., Zr, Ru) and 5d (e.g., Hf, Pt) transition metals.
- Because the 4d and 5d orbitals are more diffuse, the reduction in radius leads to smaller atomic and ionic radii, lower preferred coordination numbers and shorter metal–ligand bond lengths, and an overall shift toward more covalent bonding characteristics.
Common mistake: Confusing contraction with periodic trend of atomic number only
Concept tested: Lanthanide contraction effect on transition metals
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