Question

The lanthanide contraction is responsible for the fact that:
A.$Zn$ and $Hf$ have about the same radii
B.$Zr$ and $Yb$ have a similar oxidation state
C.$Zr$ and $Hf$ have about the same radii
D.$Zr$ and $Zn$ have a similar oxidation state

Answer 1

To answer this question, you should recall that the atomic size or the ionic radii of tri positive lanthanide ions decrease steadily due to increasing nuclear charge and electrons entering inner $\left( {n - 2} \right){\text{}}f$ orbital. This gradual decrease in the size with an increasing atomic number is called lanthanide contraction.

Complete step by step answer:
- Due to lanthanide contraction, the size for $Zr$ and $Hf$ is the same. Lanthanide contraction can be explained based on the shielding effect. In multi-electron atoms, the electrons are added in outer shells.
- The electrons already present in inner shells shield the outer electrons from nuclear charge, making them experience a lower effective charge of the nucleus. The shielding effect exerted by the inner electrons decreases in the order $s > p > d > f.$
- This f subshell poorly shields the outer electron from nuclear attraction which results in the more attractive pull of the nucleus on the outer electron and smaller size. In the case of post lanthanide elements like $Hf$ 4f subshell is filled and it is not very effective at shielding the outer shell (n = 5 and n = 6) electrons, it is similar to $Zr$.

Hence, the correct answer is option C.

Note:
Inner transition elements refer to the elements constituting the f-block are those in which 4f and 5f orbitals are progressively filled. These elements are formal members of group 3 but are shown separately as f-block of the periodic table. The elements of f-block are also called inner transition elements. There are two series of inner transition elements which are known as lanthanide or 4f series and actinoids or 5f series. The inner transition elements such as Uranium, Thorium and Protactinium are used as sources of nuclear energy.