Question

Which among the following elements shows same atomic radius?

• A. Mo & W
• B. Ti & La
• C. Ag & Ni
• D. Mn & S
• E. U & W

Hint:

Remember the three most frequently tested pairs for lanthanoid contraction:
Zr (Zirconium) and Hf (Hafnium)
Nb (Niobium) and Ta (Tantalum)
Mo (Molybdenum) and W (Tungsten)

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Generally, atomic radius increases as we move down a group in the periodic table due to the addition of new electron shells. However, in the transition elements (d-block), a phenomenon called lanthanoid contraction drastically alters this expected trend for the elements of the 5d series.
Step 2: Key Formula or Approach:
The approach involves identifying which of the given pairs consists of a 4d transition metal and the corresponding 5d transition metal immediately below it in the same group. Lanthanoid contraction causes these pairs to have nearly identical atomic radii.
Step 3: Detailed Explanation:
The lanthanoid contraction is caused by the poor shielding effect of the 14 electrons present in the \(4f\) subshell. As we move across the lanthanoid series (Ce to Lu), the increasing nuclear charge draws the electrons closer, causing a steady contraction in size.
By the time we reach the 5d transition elements (Hf, Ta, W, etc.), this significant contraction perfectly offsets the expected increase in size that should occur from adding a new principal quantum shell. Consequently, the elements of the 3rd transition series (5d) have atomic radii very similar to the elements of the 2nd transition series (4d) lying directly above them.
Let's analyze the pairs:
- A) Mo (Molybdenum) & W (Tungsten): Mo is in the 4d series (Group 6), and W is immediately below it in the 5d series (Group 6). Due to lanthanoid contraction, their radii are almost identical (\(\sim 139 \text{ pm}\)). This is a classic textbook example.
- B) Ti & La: Ti is a 3d element, La is a 5d element. They are not in the same group and differ significantly in size.
- C) Ag & Ni: Ag is 4d (Group 11), Ni is 3d (Group 10). Different groups and periods; radii are not the same.
- D) Mn & S: Mn is a transition metal, S is a p-block non-metal. Vastly different.
- E) U & W: Uranium is an actinide; Tungsten is a transition metal.
Step 4: Final Answer:
Mo and W exhibit nearly the same atomic radius due to lanthanoid contraction.