# Given below are two statement:

Question:

Given below are two statement: one is labelled as Assertion A and the other is labelled as Reason $\mathrm{R}$ :

Assertion A: Size of $\mathrm{Bk}^{3+}$ ion is less than $\mathrm{Np}^{3+}$ ion. Reason $\mathrm{R}$ :

The above is a consequence of the lanthanoid contraction. In the light of the above statements, choose the correct answer from the options given below :

1. $\mathrm{A}$ is false but $\mathrm{R}$ is true

2. Both $\mathrm{A}$ and $\mathrm{R}$ are true but $\mathrm{R}$ is not the correct explanation of $\mathrm{A}$

3. Both $A$ and $R$ are true and $R$ is the correct explanation of $\mathrm{A}$

4. $\mathrm{A}$ is true but $\mathrm{R}$ is false

Correct Option: , 4

Solution:

Size of ${ }_{97} \mathrm{Bk}^{3+}$ ion is less than that of ${ }_{93} \mathrm{~Np}^{3+}$ due to actinoid contraction.

As we know that in a period from left to right ionic radius decreases and in actinide series it is due to actinoid contraction.