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 :
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.