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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3pt108

Which statements about the lanthanoid contraction and its effects are correct?

AIt arises because $4 f$ electrons shield the nuclear charge poorly, so $Z_{eff}$ rises along the series.
BIt makes the radius of Hf about one full shell larger than that of Zr.
CIt causes the basic strength of $Ln (OH) X_{3}$ to increase from La to Lu.
DIt makes $Y X^{3 +}$ close in size to the heavier $Ln X^{3 +}$ ions such as $Ho X^{3 +}$ .

Answer: A, D

Poor

4 f

shielding raises

Z_{eff}

, so

Ln X^{3 +}

radii fall from La to Lu and

Y X^{3 +}

(90.0 pm) matches

Ho X^{3 +}

(90.1 pm). Smaller cations are more polarising, so hydroxide basicity decreases. The same contraction cancels the expected growth between Zr and Hf, so they are nearly equal in size.

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