Which of the following pair of elements does not have same size ?

Q: Which of the following pair of elements does not have same size ?

Which of the following pair of elements does not have same size ?

Question

Accepted Answer

Option 1 is the correct answer because Nb (Niobium) and Os (Osmium) do not have the same size. This difference in size can be attributed to their positions in the periodic table and their respective atomic structures.

1. **Explanation for Option 1 (Nb and Os):**
   - Niobium (Nb) is located in Group 5 and Period 5 of the periodic table, while Osmium (Os) is in Group 8 and Period 6. The atomic radius of an element generally decreases across a period and increases down a group. However, Nb and Os are not in the same group or period, leading to a significant difference in their atomic sizes.
   - Niobium has an atomic number of 41, and Osmium has an atomic number of 76. The increase in atomic number and the presence of more electron shells in Osmium contribute to its larger atomic size compared to Niobium.

2. **Explanation for Option 2 (Zr and Hf):**
   - Zirconium (Zr) and Hafnium (Hf) are known to have very similar atomic sizes due to the phenomenon known as the "lanthanide contraction." Both elements are in Group 4, with Zr in Period 5 and Hf in Period 6. The lanthanide contraction causes the atomic radius of Hf to be very similar to that of Zr, despite being in a different period.

3. **Explanation for Option 3 (Pd and Pt):**
   - Palladium (Pd) and Platinum (Pt) are also affected by the lanthanide contraction. Both elements belong to Group 10, with Pd in Period 5 and Pt in Period 6. The lanthanide contraction results in Pt having a similar atomic size to Pd, despite being in a higher period.

4. **Explanation for Option 4 (Mo and W):**
   - Molybdenum (Mo) and Tungsten (W) are in Group 6, with Mo in Period 5 and W in Period 6. Similar to the previous cases, the lanthanide contraction causes W to have an atomic size comparable to Mo, even though it is in a different period.

In conclusion, Option 1 is correct because Nb and Os do not have the same size due to their different positions in the periodic table and the absence of factors like the lanthanide contraction that would otherwise equalize their atomic sizes.