Question

Which among the following properties of lanthanoids is NOT true?

• A. These are good conductors of heat and electricity.
• B. These are strongly paramagnetic.
• C. These all are non-radioactive.
• D. These have coordination number greater than 6.

Hint:

Promethium (Pm, atomic number 61) is famously known as the only synthetic, fully radioactive element inside the core lanthanoid series. Remembering this exception lets you spot the false claim instantly.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question requires us to identify the statement that is factually incorrect regarding the physical or chemical properties of the lanthanoid series elements.

Step 2: Detailed Explanation:
Let's analyze the properties of lanthanoids across the given choices:
Lanthanoids are typical metallic elements, making them highly efficient conductors of both heat and electricity.
Most tripositive lanthanoid ions ($\text{Ln}^{3+}$) possess unpaired electrons in their deep-lying $4\text{f}$ subshells, which induces strong paramagnetic properties (except for $\text{La}^{3+}$ and $\text{Lu}^{3+}$).
Due to their large ionic radii, lanthanoids easily accommodate crowded ligand spheres, frequently exhibiting coordination numbers greater than 6 (commonly 7, 8, or 9).
While almost all naturally occurring lanthanoids are stable and non-radioactive, there is a prominent exception:

Promethium (Pm) is entirely radioactive and does not occur naturally in significant quantities.
Because promethium is radioactive, the absolute statement that "these all are non-radioactive" is false.

Step 3: Final Answer: The incorrect property statement corresponds to option (C).