Neet Solved Paper 2015 Question 6
Question: Because of lanthanoid contraction, which of the following pairs of elements have nearly same atomic radii? (Numbers 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) $
Show Answer
Answer:
Correct Answer: C
Solution:
Because of the lanthanoid contraction $ Zr $ (atomic radii 160 pm) and $ Hf $ (atomic radii 158. pm) have nearly same atomic radii.
Lanthanoids include the elements from lanthanum La (Z = 57) to lutetium Lu (Z = 71).
zirconium Zr (40) belong to the second transition series (4d) and Hf (72) belongs to third transition series (5d).
Lanthanoid contraction is associated with the intervention of the 4f orbitals which are filled before the 5d-series of elements starts.
The filling of 4f if orbitals before 5d-orbitals results in regular decrease in atomic radii which compensates the expected increase in atomic size with increasing atomic number. As a result of this lanthanoid contraction, the elements of second and third transition series have almost similar atomic radii.
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