Which of the following pairs of elements have alomost similar atomic radii ?

Options

A. `Zr, Hf`
B. `Mo, W`
C. `Co, Ni`
D. `Nb, Ta`

Answer

To determine which pairs of elements have almost similar atomic radii, we can analyze the given options based on their positions in the periodic table and the concept of lanthanide contraction.

### Step-by-Step Solution:

1. **Identify the Elements in Each Pair**:
- Pair 1: Zirconium (Zr) and Hafnium (Hf)
- Pair 2: Molybdenum (Mo) and Tungsten (W)
- Pair 3: Cobalt (Co) and Nickel (Ni)
- Pair 4: Nobelium (No) and Tantalum (Ta)

2. **Understand the Concept of Atomic Radii**:
- Atomic radius is the distance from the nucleus of an atom to the outermost shell of electrons. It can vary based on the element's position in the periodic table.

3. **Analyze Pair 1: Zirconium and Hafnium**:
- Zirconium (Zr) is in the 4D transition series, while Hafnium (Hf) is in the 5D transition series.
- Due to the lanthanide contraction, the atomic radii of Zr and Hf are almost similar. The presence of lanthanides causes a decrease in the size of the 5D elements.

4. **Analyze Pair 2: Molybdenum and Tungsten**:
- Molybdenum (Mo) is in the 4D series, and Tungsten (W) is in the 5D series.
- Similar to the first pair, the atomic radii of Mo and W are also affected by lanthanide contraction, leading to similar sizes.

5. **Analyze Pair 3: Cobalt and Nickel**:
- Both Cobalt (Co) and Nickel (Ni) are in the 3D transition series.
- Elements in the same series generally have similar atomic radii due to their similar electron configurations.

6. **Analyze Pair 4: Nobelium and Tantalum**:
- Nobelium (No) is in the actinides, while Tantalum (Ta) is in the 5D series.
- The atomic radii of these two elements may not be as similar as the previous pairs due to their different series and the influence of f-orbitals.

7. **Conclusion**:
- Pairs 1 (Zr and Hf), 2 (Mo and W), and 3 (Co and Ni) have almost similar atomic radii due to their respective positions in the periodic table and the effects of lanthanide contraction.
- Pair 4 (No and Ta) does not have similar atomic radii.

### Final Answer:
The pairs of elements that have almost similar atomic radii are:
- Zirconium and Hafnium
- Molybdenum and Tungsten
- Cobalt and Nickel