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A major consequence of lanthanoid contraction is:
ALanthanoids are gases
BThe actinoids exist only as gases
CThe lanthanoids are radioactive
DThe 4d (second) and 5d (third) transition series have nearly identical atomic radii, making chemically similar pairs like Zr/Hf and Nb/Ta hard to separate
Answer & Solution
Correct answer: D. The 4d (second) and 5d (third) transition series have nearly identical atomic radii, making chemically similar pairs like Zr/Hf and Nb/Ta hard to separate
The cumulative 4f contraction cancels the expected 4d→5d size increase → **Zr (160 pm) ≈ Hf (159 pm); Nb ≈ Ta; Mo ≈ W**. These pairs co-occur in nature and are notoriously hard to separate by classical methods.
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