Lanthanoid contraction will not have any influence when we compare size of :-

Options

A. Zr & Hf
B. Y & La
C. Mo & W
D. Tc & Re

Answer

To solve the question regarding the influence of lanthanoid contraction on the size comparison of certain elements, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in the size of the lanthanide ions as we move from lanthanum (La) to lutetium (Lu) in the periodic table. This occurs due to the poor shielding effect of the f-electrons, leading to an increased effective nuclear charge.

2. **Identify the Elements in Each Option**:
- Option 1: Zirconium (Zr) and Hafnium (Hf)
- Option 2: Yttrium (Y) and Lanthanum (La)
- Option 3: Molybdenum (Mo) and Tungsten (W)
- Option 4: Technetium (Tc) and Rhenium (Re)

3. **Analyze Each Pair**:
- **Option 1**: Zr and Hf are in the d-block and are influenced by the lanthanoid contraction due to their proximity to the lanthanides in the periodic table.
- **Option 2**: Y and La are also influenced by lanthanoid contraction since La is a lanthanide and Y is in the same period.
- **Option 3**: Mo and W are d-block elements and will also be influenced by the contraction as they are in the same group.
- **Option 4**: Tc and Re are both d-block elements but are not directly influenced by lanthanoid contraction as they are further down the periodic table and not in the same group as the lanthanides.

4. **Conclusion**:
- The pair that will not be influenced by lanthanoid contraction when comparing sizes is **Option 4: Technetium (Tc) and Rhenium (Re)**.

### Final Answer:
The correct answer is **Option 4: Technetium and Rhenium**.

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