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) $Zn$ and $Zr$ have the same oxidation state

Answer 1

We have to remember that in periodic tables various kinds of elements based on the shape of the orbitals. There are $s$ , $p$ , $d$ and $f$ block elements. The $f$ block elements are further classified as $4f$ and $5f$ block elements. The $4f$ block element is known as lanthanide series. Lanthanide starting from cerium to lutetium. These 14 elements are commonly known as lanthanide series.

Complete step by step answer:
We have to know that in a periodic table, moving from left to right, the atomic size of the element decreases because the effective nuclear charge of the atom is increased.
But in $4f$ block elements, atomic size is not decreased from one element to another element due to poor shielding effect of the inner electrons, even one electron and one neutron added in atom also. This kind of contraction in $4f$ block elements is popularly known as lanthanide contraction.
The fourteen elements in $4f$ block elements are influenced by lanthanide contraction in periodic table.
We need to know that the atomic radii of zirconium and hafnium are nearly close. The atomic radius of zirconium is $159pm$ and the atomic radius of hafnium is $156pm$. Because of lanthanide contraction.
As we know that in general periodic trend wise, the increase period increases the size of the atom. The atomic number of Zirconium is $40$ in $5$ period and hafnium is $72$ in $6$ period. The atomic mass of zirconium is $91.22g$ and hafnium is $178.49g$ . The density of zirconium and hafnium is $6.51g/c{m^3}$ and $13.35g/c{m^3}$ .
We need to know that all other properties of zirconium and hafnium are gradually increasing, but atomic radius only comes nearly the same due to lanthanide contraction.
According to the lanthanide contraction, option C is correct.
$Zr$ and $Hf$ have about the same radius
Option A is not correct, because $Zn$ and $Y$ have not the same radii
Option B is not correct, because $Zr$ and $Nb$ have not similar oxidation state
Option D is not correct, because $Zn$ and $Zr$ have not the same oxidation state.

So, the correct answer is Option C.

Note: We need to know that in zirconium and hafnium atomic radius are nearly the same due to the lanthanide contraction. Due to this size of the atoms, chemical properties of zirconium and hafnium are similar. Reactivity of each atom depends on the size. The similar size of these atoms lattices energy and solvent energy of the both atoms also similar. The stability constants of both atoms are nearly the same. The common oxidation state of lanthanide series is $+ 3$ .