Indium and thallium of Ill group have nearly similar atomic radii due to poor screening effect shown by f-electrons in the

Options

A. Penultimate shell of thallium
B. Anti penultimate shell of indium
C. Anti penultimate shell of thallium
D. Penultimate shell of indium

Answer

To solve the question regarding the similar atomic radii of Indium (In) and Thallium (Tl) in Group III, we need to understand the concept of atomic radii, screening effect, and the electronic configurations of these elements.

### Step-by-Step Solution:

1. **Identify the Elements and Their Group**:
- Indium (In) and Thallium (Tl) belong to Group III of the periodic table, also known as the Boron family.

2. **Write the Electronic Configurations**:
- Indium (In) has an atomic number of 49. Its electronic configuration is:
\[ \text{In: [Kr] 4d}^{10} 5s^{2} 5p^{1} \]
- Thallium (Tl) has an atomic number of 81. Its electronic configuration is:
\[ \text{Tl: [Xe] 4f}^{14} 5d^{10} 6s^{2} 6p^{1} \]

3. **Identify the Shells**:
- For Indium:
- **Ultimate shell**: 5s² 5p¹ (5th shell)
- **Penultimate shell**: 4d¹⁰ (4th shell)
- **Anti-penultimate shell**: 4s (3rd shell)
- For Thallium:
- **Ultimate shell**: 6s² 6p¹ (6th shell)
- **Penultimate shell**: 5d¹⁰ (5th shell)
- **Anti-penultimate shell**: 4f¹⁴ (4th shell)

4. **Understand the Screening Effect**:
- The screening effect refers to the ability of inner electrons to shield the outer electrons from the full effect of the nuclear charge.
- In Thallium, the presence of 14 f-electrons in the 4f subshell leads to poor shielding of the outer electrons (6s and 6p). This is known as the "inert pair effect."

5. **Compare Atomic Radii**:
- The atomic radius of Indium is approximately 167 pm, while that of Thallium is about 170 pm.
- Despite the difference, these values are relatively close, indicating that the atomic radii of Indium and Thallium are similar.

6. **Conclusion**:
- The nearly similar atomic radii of Indium and Thallium can be attributed to the poor screening effect caused by the f-electrons in the anti-penultimate shell of Thallium. This results in a reduced effective nuclear charge felt by the outermost electrons, leading to similar atomic sizes.

### Final Answer:
Indium and Thallium have nearly similar atomic radii due to the poor screening effect shown by the f-electrons in the anti-penultimate shell of Thallium.

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