Question

Number of colourless lanthanoid ions among the following is ____.
\[ \text{Eu}^{3+}, \, \text{Lu}^{3+}, \, \text{Nd}^{3+}, \, \text{La}^{3+}, \, \text{Sm}^{3+} \]

Answer 1

Correct Answer: 2

Lanthanoid ions are known to be colorful due to electronic transitions within their 4f orbitals, except when these orbitals are either empty or completely filled. A key factor determining whether an ion is colorless or colorful is the presence of unpaired electrons in the f orbitals. Let's analyze each ion provided:

• \(\text{Eu}^{3+}\): The electron configuration is \([\text{Xe}] \, 4f^6\). There are unpaired electrons, giving rise to color.
• \(\text{Lu}^{3+}\): The electron configuration is \([\text{Xe}] \, 4f^{14}\). There are no unpaired electrons; it is colorless.
• \(\text{Nd}^{3+}\): The electron configuration is \([\text{Xe}] \, 4f^3\). There are unpaired electrons, giving rise to color.
• \(\text{La}^{3+}\): The electron configuration is \([\text{Xe}] \, 4f^0\). There are no electron transitions possible, making it colorless.
• \(\text{Sm}^{3+}\): The electron configuration is \([\text{Xe}] \, 4f^5\). There are unpaired electrons, giving rise to color.

Based on this analysis, the colorless ions are \(\text{Lu}^{3+}\) and \(\text{La}^{3+}\). Hence, the number of colorless lanthanoid ions is 2.

Answer 2

Step 1: Analyze electronic configurations

1. La³⁺: [Xe]4f⁰ (No unpaired electrons, colourless).
2. Lu³⁺: [Xe]4f¹⁴ (No unpaired electrons, colourless).
3. Nd³⁺: [Xe]4f³ (Unpaired electrons, coloured).
4. Sm³⁺: [Xe]4f⁵ (Unpaired electrons, coloured).
5. Eu³⁺: [Xe]4f⁶ (Unpaired electrons, coloured).

Step 2: Count colourless ions

Lanthanide ions that are colourless: La³⁺ and Lu³⁺.

Final Answer: 2.