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The contraction in atomic radii along the lanthanoid series ($\mathrm{La}$ to $\mathrm{Lu}$) is called lanthanoid contraction. It arises mainly because:
AThe added 4f electrons shield the nuclear charge very effectively, so atomic size increases
BThe added 4f electrons shield the nuclear charge poorly, so the effective nuclear charge on outer electrons increases steadily across the series
CThe added 4f electrons cause expansion of the 6s orbital
DRelativistic effects dominate
Answer & Solution
Correct answer: B. The added 4f electrons shield the nuclear charge poorly, so the effective nuclear charge on outer electrons increases steadily across the series
The 4f orbitals are diffuse and penetrate poorly to the nucleus, so they shield outer (5d, 6s) electrons very inefficiently from the increasing nuclear charge across the lanthanoid series. The net result is a steady, small contraction in atomic and ionic radii from La to Lu. A consequence is that the radii of 4d and 5d transition elements in the same group are very similar (e.g. Zr ≈ Hf), which makes their separation chemically difficult.
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