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 exhibit color due to electronic transitions within their 4f orbitals, unless these orbitals are empty or fully occupied. The presence of unpaired electrons in the f orbitals is crucial for color. An examination of each ion is provided:

• \(\text{Eu}^{3+}\): Electron configuration \([\text{Xe}] \, 4f^6\). Unpaired electrons are present, resulting in color.
• \(\text{Lu}^{3+}\): Electron configuration \([\text{Xe}] \, 4f^{14}\). No unpaired electrons are present; it is colorless.
• \(\text{Nd}^{3+}\): Electron configuration \([\text{Xe}] \, 4f^3\). Unpaired electrons are present, resulting in color.
• \(\text{La}^{3+}\): Electron configuration \([\text{Xe}] \, 4f^0\). No electron transitions are possible, rendering it colorless.
• \(\text{Sm}^{3+}\): Electron configuration \([\text{Xe}] \, 4f^5\). Unpaired electrons are present, resulting in color.

Consequently, the colorless ions identified are \(\text{Lu}^{3+}\) and \(\text{La}^{3+}\). The count of colorless lanthanoid ions is therefore 2.