Given below are two statement: one is labelled as Assertion $\mathbf{A}$ and the other is labelled as Reason R: Assertion A: Size of $\ce{Bk^{3+}}$ ion is less than $\ce{Np^{3+}}$ ion. Reason 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:

(B) $\mathbf{A}$ is true but $\mathbf{R}$ is false

(A) Both $\mathbf{A}$ and $\mathbf{R}$ are true but $\mathbf{R}$ is not the correct explanation of $\mathbf{A}$

(C) Both $\mathbf{A}$ and $\mathbf{R}$ are true and $\mathbf{R}$ is the correct explanation of $\mathbf{A}$

(D) $\mathbf{A}$ is false but $\mathbf{R}$ is true

JEE Main · 2021 · 16 Mar 2021 · Shift 1 · d and f Block Elements

Given below are two statement: one is labelled as Assertion

A

and the other is labelled as Reason R:
Assertion A: Size of

Bk X^{3 +}

ion is less than

Np X^{3 +}

ion.
Reason 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:

(A)Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$
(B) $A$ is true but $R$ is false✓ Correct
(C)Both $A$ and $R$ are true and $R$ is the correct explanation of $A$
(D) $A$ is false but $R$ is true

Answer: (B)

Solution

Berkelium (

Z = 97

) stands further along the actinoid series than neptunium (

Z = 93

). The electrons added between them enter

5 f

orbitals, which screen the nucleus poorly, so the effective nuclear charge acting on the outer shell grows and the ionic radius shrinks;

B k^{3 +}

is accordingly smaller than

N p^{3 +}

.
This steady contraction along the

5 f

series is the actinoid contraction. The lanthanoid contraction is the parallel effect produced by filling the

4 f

shell across the lanthanoids, and it does not govern the sizes of actinoid ions.
Assertion A is true but Reason R is false.

JEE Main · 2021 · 16 Mar 2021 · Shift 1 · Single correct option

Practise more: all JEE Main PYQs on d and f Block Elements →

JEE Main · 2021 · 16 Mar 2021 · Shift 1 · d and f Block Elements

Solution

More JEE Main questions on d and f Block Elements