Question

Lanthanoids are known for:

• A. Variable oxidation states only
• B. Formation of coloured ions
• C. Absence of magnetic properties
• D. Non-metallic character

Hint:

The characteristic colours of lanthanoid ions depend on the number of \(f\)-electrons.
Interestingly, \(x\)-electrons and \((14-x)\)-electrons in the \(4f\) shell often produce similar colours.
For example, \(\text{Pr}^{3+}\) (\(4f^2\)) and \(\text{Tm}^{3+}\) (\(4f^{12}\)) both exhibit a green colour.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The objective is to identify a prominent characteristic feature of the lanthanoid elements from the given options.

Step 2: Key Formula or Approach:
Lanthanoids have partially filled \(4f\) subshells, which enables electronic transitions that absorb and emit visible light, resulting in coloured compounds.

Step 3: Detailed Explanation:

• Coloured Ions: Most trivalent lanthanoid ions (\(\text{Ln}^{3+}\)) are coloured in both solid and aqueous states.
  
• Reason for Colour: The colour is due to the absorption of light in the visible region, causing \(f\text{–}f\) electronic transitions because of the presence of unpaired electrons in the \(4f\) subshell.
  
• Analyzing Option (A) - Variable oxidation states only: Lanthanoids do show oxidation states of \(+2\) and \(+4\) besides their stable \(+3\) state, but the term "only" makes this option incorrect.
  
• Analyzing Option (C) - Absence of magnetic properties: Lanthanoids exhibit magnetic properties due to the unpaired electrons in their \(4f\) orbitals. Thus, they are paramagnetic, not diamagnetic.
  
• Analyzing Option (D) - Non-metallic character: Lanthanoids are highly electropositive and silvery-white metals, so they have a strong metallic character rather than a non-metallic one.
  

Step 4: Final Answer:
The key property lanthanoids are known for is the formation of coloured ions, which corresponds to option (B).