Question

The lanthanide contraction is responsible for the fact that ________.

• A. Zn and Y have about the same radii
• B. Zr and Nb have similar oxidation state
• C. Zr and Hf have about the same radii
• D. Zr and Zn have the same oxidation state

Answer 1

Answer: (C)

Zr and Hf have about the same radii

The elements following the lanthanides in the periodic table are influenced by the lanthanide contraction. For example, the atomic radius of the metal zirconium, Zr, (a period-5 transition element) is 159 pm and that of hafnium, Hf, (the corresponding period-6 element) is 156 pm.

The radii are very similar even though the number of electrons increases from 40 to 72 and the atomic mass increases from 91.22 to 178.49 g/mol. The increase in mass and the unchanged radii lead to a steep increase in density from 6.51 to 13.35 g/cm${}^3$.

Zirconium and hafnium, therefore, have very similar chemical behaviour, having closely similar radii and electron configurations. Radius-dependent properties such as lattice energies, solvation energies, and stability constants of complexes are also similar.