Question

The incorrect statement among the following is :

• A. Lanthanoids are good conductors of heat and electricity.
• B. Actinoids are highly reactive metals, especially when finely divided.
• C. Actinoid contraction is greater for element-to-element than Lanthanoid contraction.
• D. Most of the trivalent Lanthanoid ions are colourless in the solid state.

Hint:

Color in Lanthanoids arises due to \(f-f\) transitions. Ions with completely empty, half-filled, or completely filled \(f\)-orbitals are generally colorless.

Answer 1

The Correct Option is D

Concept: Lanthanoids and Actinoids belong to the \(f\)-block elements and exhibit characteristic properties such as variable oxidation states, contraction, color, magnetic behavior, and high metallic character.

Step 1: Checking statement (a).
Lanthanoids are metallic elements. They possess free electrons responsible for metallic conductivity. Therefore they conduct:
• Heat
• Electricity efficiently. Hence statement (a) is correct.

Step 2: Checking statement (b).
Actinoids are highly electropositive metals. Finely divided actinoids react rapidly because surface area increases significantly. Hence statement (b) is correct.

Step 3: Checking statement (c).
Actinoid contraction is larger because:
• \(5f\)-electrons shield very poorly
• effective nuclear charge increases strongly Thus contraction becomes greater than lanthanoid contraction. Hence statement (c) is correct.

Step 4: Checking statement (d).
Most trivalent lanthanoid ions are actually colored due to \(f-f\) electronic transitions. Only ions having: \[ f^0,\; f^7,\; f^{14} \] configurations are generally colorless. Examples: \[ La^{3+},\; Gd^{3+},\; Lu^{3+} \] Since most lanthanoid ions are colored, statement (d) is incorrect. Final Answer: \[ \boxed{\text{Most of the trivalent Lanthanoid ions are colourless in the solid state}} \]