Lanthanoid Contraction
The steady decrease in atomic and ionic radii across the lanthanoid series leads to which important consequence for the elements that follow them?
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Solution
Nearly identical radii of corresponding 4d and 5d transition elements
Lanthanoid contraction is the gradual decrease in the atomic and ionic radii of the lanthanoid elements as the atomic number increases across the series, caused by the poor shielding of the nuclear charge by the diffuse 4f electrons. An important consequence is that the elements of the third transition series, the 5d elements that come after the lanthanoids, have radii almost identical to those of the corresponding elements of the second transition series, the 4d elements, because the contraction offsets the expected increase in size. This makes pairs such as zirconium and hafnium very similar in size and difficult to separate. The option of a large size difference is the opposite of the actual effect. The option of an increase in radii down the group ignores the contraction. The option of complete loss of metallic character is unrelated. This consequence is a key NCERT f-block concept. It is worth emphasising that this is not a special case but a representative example of how lanthanoid contraction operates throughout d- and f-block elements. Plausibility check: the famously similar sizes and chemistry of zirconium and hafnium are a direct result of lanthanoid contraction, confirming the answer.
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About This Question
- Subject
- chemistry
- Chapter
- d- and f-block elements
- Topic
- lanthanoid contraction
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Nearly identical radii of corresponding 4d and 5d transition elements
Lanthanoid contraction is the gradual decrease in the atomic and ionic radii of the lanthanoid elements as the atomic number increases across the series, caused by the poor shielding of the nuclear charge by the diffuse 4f electrons. An important consequence is that the elements of the third transition series, the 5d elements that come after the lanthanoids, have radii almost identical to those of the corresponding elements of the second transition series, the 4d elements, because the contraction offsets the expected increase in size. This makes pairs such as zirconium and hafnium very similar in size and difficult to separate. The option of a large size difference is the opposite of the actual effect. The option of an increase in radii down the group ignores the contraction. The option of complete loss of metallic character is unrelated. This consequence is a key NCERT f-block concept. It is worth emphasising that this is not a special case but a representative example of how lanthanoid contraction operates throughout d- and f-block elements. Plausibility check: the famously similar sizes and chemistry of zirconium and hafnium are a direct result of lanthanoid contraction, confirming the answer.
This medium difficulty chemistry question is from the chapter d- and f-block elements, covering the topic of lanthanoid contraction. It appeared in the 2025 exam.
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