Question

The Lanthanide contraction is responsible for the fact that ………...?
A.) Zn and Y have about the same radii
B.) Zr and Zb have similar oxidation state
C.) Zr and Hf have about the same radii
D.) Zr and Zn have same oxidation state

Answer 1

Hint: As we move from left to right in the lanthanide series from lanthanum (57) to lutetium (71), size of the lanthanide ions ${{M}_{3}}^{+}$ decreases with increase in atomic number. This phenomenon is known as lanthanide contraction.

Complete step by step solution:

With increasing atomic number in the lanthanide series, the number of 4f electrons increases as well as the effective nuclear charge.
As there will be an increase in the number of protons in the nucleus with increasing atomic number, there will be an effective nuclear charge.
We know that the f electrons have a poor shielding effect (order of shielding effect s >p > d> f) due to which the outer electrons will be attracted towards the nucleus, this causes a contraction in the electron cloud of the 4f electron. Thereby, reducing the size.
Now, due to the poor shielding effect of the 4f electrons, the size of the 2nd and 3rd series elements of the transition series are almost the same size and they also resemble in their properties.
Therefore, the sizes of Hf-Zr, Nb-Ta, Mo-W are almost the same.

Some more consequences of lanthanide contraction are:
With a regular decrease in the size of lanthanide ions with increased atomic number, the size of the cation decreases. Thereby, increasing the polarizing power of the cation. This also reduces the basic strength hydroxides and oxides.
There will be a regular decrease in the tendency to act as a reducing agent with increasing atomic number.
Due to the contraction the elements occur together as natural minerals and are difficult to separate.
Thus, from the above statements we can conclude that Zr and Hf have about the same radii.

So, the correct option is (c).

Note: Some general properties about lanthanides to remember :
Lanthanides are a type of rare earth metals having similar size and generally have +3 oxidation state.
They are silvery white metals which tarnish when exposed to air forming oxides.
They have high melting and boiling points.
They are reactive in nature, mainly reacting with water to liberate hydrogen.