Question

The correct statements among the following are:
A. Mo(VI) and W(VI) are less stable than Cr(VI).
B. Ce⁴⁺ and Tb⁴⁺ are oxidant while Eu²⁺ and Yb²⁺ are reductant.
C. Cm and Am have seven unpaired electrons.
D. Actinoid contraction is greater from element to element than lanthanoid contraction.
Choose the correct answer:

• A. A and B Only
• B. C and D Only
• C. B and D Only
• D. A and C Only

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Stability of oxidation states in d-block elements, redox behavior of lanthanoids, electronic configurations of actinoids, and the nature of internal screening (shielding) are key to identifying correct chemical properties.

Step 2: Key Formula or Approach:
1. Stability of higher oxidation states increases down the group in d-block. 2. Lanthanoids prefer the +3 state; those in +4 are oxidizing, and those in +2 are reducing. 3. Compare shielding of $4f$ vs $5f$ orbitals.

Step 3: Detailed Explanation:
1. Statement A analysis: In Group 6, the stability of the +6 oxidation state increases down the group ($Cr<Mo<W$). Thus, $Mo(VI)$ and $W(VI)$ are more stable than $Cr(VI)$, making Statement A false. 2. Statement B analysis: $Ce^{4+}$ and $Tb^{4+}$ tend to gain an electron to reach the stable +3 state (oxidants). $Eu^{2+}$ and $Yb^{2+}$ tend to lose an electron to reach +3 (reductants). This is true. 3. Statement C analysis: $Am$ ($[Rn] 5f^7 7s^2$) has 7 unpaired electrons, but $Cm$ ($[Rn] 5f^7 6d^1 7s^2$) has 8 unpaired electrons ($7$ in $f$ and $1$ in $d$). Thus, Statement C is false. 4. Statement D analysis: $5f$ electrons provide poorer shielding than $4f$ electrons. Consequently, the effective nuclear charge increases more sharply, making actinoid contraction greater than lanthanoid contraction. This is true.

Step 4: Final Answer:
Statements B and D are correct.