Given below are two statement : one is labelled as Assertion A and the other is labelled as Reason R :
Assertion A : Size of Bk³⁺ ion is less than Np³⁺ 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 :

Solution

Chemistry ma’am · 4 steps

Bk and Np are both actinoids, so the shrinking here is the actinoid contraction, not the lanthanoid one.

1. Place both ions. Np has Z = 93 and Bk has Z = 97. Both sit in the 5f series, the actinoids. Bk comes four places after Np.

   Ion	Atomic number	Series	Configuration
   Np³⁺	93	actinoid	[Rn] 5f⁴
   Bk³⁺	97	actinoid	[Rn] 5f⁸

2. Check Assertion A. From Np to Bk the nuclear charge rises by 4. The extra electrons go into 5f orbitals, which shield poorly, so the outer electrons are pulled in harder. Bk³⁺ is therefore smaller than Np³⁺, and A is true.
3. Check Reason R. The lanthanoid contraction is the steady decrease in size across the 4f series, from La to Lu. Neither Np nor Bk is a lanthanoid. The shrinking here is the actinoid contraction, so R is false.
4. So. A is true but R is false, which is option (c).

The idea to keep. Always place the ions first. A 4f series ion shrinks by the lanthanoid contraction and a 5f series ion by the actinoid contraction. Same physics, but in JEE the name has to match the series.

Answer from the NTA final answer key. Solution by PrepWiser.