Given below are two statements.
One is labelled as Assertion (A) and the other is labelled as Reason

Q: Why does the size decrease across the actinide series?

Due to the poor shielding effect of $5f$ electrons, the effective nuclear charge ($Z_{\text{eff}}$) increases as protons are added, pulling the valence shell closer to the nucleus.

Q: What is the difference between lanthanoid and actinoid contraction?

Lanthanoid contraction occurs in the $4f$ series (Lanthanides), while actinoid contraction occurs in the $5f$ series (Actinides). The underlying physics is the same, but the terminology is specific to the electron shell being filled.

Q: Why is actinoid contraction greater from element to element than lanthanoid contraction?

The $5f$ orbitals are more diffused and spread out than $4f$ orbitals, leading to even poorer shielding of the nuclear charge.

Q: How do I remember which elements are actinides?

Actinides start from Actinium ($Z=89$) or Thorium ($Z=90$) up to Lawrencium ($Z=103$). Neptunium ($93$) and Berkelium ($97$) fall right in the middle of this range.

Q: Is the assertion completely correct on its own?

Yes, the physical fact stated in the assertion ($\text{Bk}^{3+}$ is smaller than $\text{Np}^{3+}$) is perfectly correct. The question only becomes tricky when evaluating the specific wording of the reason.

Question

Given below are two statements.
One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) Size of $	ext{Bk}^{3+}$ ion is less than $	ext{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

Options:

(A) A is false but R is true.

(B) Both A and R are true but R is not the correct explanation of (A)

(C) Both A and R are true and R is the correct explanation of (A)

(D) A is true but R is false.

Sigma Percentile · Accepted Answer

### The Actinide Lineup

When tackling questions on the f-block elements, the first step is always to orient yourself on the periodic table. The question presents us with two elements: Neptunium ($	ext{Np}$) and Berkelium ($	ext{Bk}$).

If you recall the actinide series, which sits at the very bottom of the periodic table, it begins with Actinium ($Z=89$) and ends with Lawrencium ($Z=103$). Neptunium has an atomic number of $93$, and Berkelium has an atomic number of $97$. Because $97 > 93$, Berkelium lies to the right of Neptunium in the actinide series.

### The Phenomenon of Actinoid Contraction

As we move from left to right across the actinide series, the atomic number increases, meaning protons are being added to the nucleus. Simultaneously, electrons are being added to the $5f$ subshell.

Here is the critical physical reality: $5f$ electrons are highly diffused and offer **terrible shielding** against the nuclear charge. Because they fail to effectively screen the outer electrons from the growing positive charge of the nucleus, the effective nuclear charge ($Z_{	ext{eff}}$) increases steadily. This increased inward pull causes the valence shell to shrink.

This steady, gradual decrease in atomic and ionic radii across the $5f$ series is known as **Actinoid Contraction**. Therefore, following this trend, the ionic radius of $	ext{Bk}^{3+}$ must be smaller than that of $	ext{Np}^{3+}$. The Assertion (A) is absolutely true.

### Catching the Trap

Now, let's look at the Reason (R). It states that this size difference is a consequence of the **lanthanoid contraction**.

Did you catch the trap? While the physical mechanism (poor shielding of f-electrons) is identical, the *name* of the phenomenon is strictly tied to the series. Lanthanoid contraction occurs in the $4f$ series (the lanthanides). Because Neptunium and Berkelium are filling the $5f$ subshell, the correct term is **actinoid contraction**.

Examiners love to test your attention to detail with these subtle word swaps. Because the reason uses the wrong terminology for the series in question, Reason (R) is false.

**Bonus Insight:** Actinoid contraction is actually more pronounced from element to element than lanthanoid contraction because the $5f$ orbitals are even more diffused than the $4f$ orbitals, leading to even poorer shielding!

Select Answer:

Visualized Solution

$Actinide Series$

$Shielding Effect$

$Actinoid Contraction$

$Comparing Sizes$

$Evaluating Reason$

$Conclusion$

The Sigma Insight: Inner Transition Elements

The Actinide Lineup

The Phenomenon of Actinoid Contraction

Catching the Trap

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