Because of lanthnoid contraction, which of the following pairs of elements have nearly same atomic radii ? (Number in the parenthesis are atomic numbers)

Options

A. Ti(22) and Zr(40)
B. Zr(40) and Nb(41)
C. Zr (40)and Hf(72)
D. Zr(40) and Ta(73)

Explanation

To determine which pairs of elements have nearly the same atomic radii due to lanthanide contraction, we need to understand the concept of lanthanide contraction and how it affects the atomic sizes of elements in the periodic table.

### Step-by-Step Solution:

1. **Understanding Lanthanide Contraction**:
- Lanthanide contraction refers to the gradual decrease in the size of the lanthanide elements (atomic numbers 57 to 71) as we move from lanthanum (La) to lutetium (Lu). This is due to the increasing nuclear charge with the addition of protons, which pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

2. **Identifying the Transition Elements**:
- The transition elements are divided into different series. The first transition series includes elements from Scandium (Sc, atomic number 21) to Zinc (Zn, atomic number 30). The second transition series includes elements from Yttrium (Y, atomic number 39) to Cadmium (Cd, atomic number 48). The third transition series includes elements from Hafnium (Hf, atomic number 72) to Copernicium (Cn, atomic number 112).

3. **Comparing Atomic Radii**:
- Due to lanthanide contraction, the atomic radii of the second transition series elements are comparable to those of the third transition series elements. Therefore, we need to find pairs of elements where one belongs to the second transition series and the other belongs to the third transition series.

4. **Analyzing the Given Options**:
- Let's analyze the provided pairs:
- **Option 1: Ti (22) and Zr (40)** - Both are in the second transition series, so this is incorrect.
- **Option 2: Zr (40) and Nb (41)** - Both belong to the same transition series, so this is incorrect.
- **Option 3: Zr (40) and Hf (72)** - Zr is in the second transition series and Hf is in the third transition series. This pair is correct due to lanthanide contraction.
- **Option 4: Zr (40) and Ta (73)** - This pair is incorrect as they belong to different series, but they are not comparable due to the diagonal relationship.

5. **Conclusion**:
- The correct answer is **Option 3: Zr (40) and Hf (72)**, as they have nearly the same atomic radii due to lanthanide contraction.