JEE Main · ChemistryMedium

$Identify the correct order of ionic radii for the following lanthanoid ions: La^{3 +}, Eu^{3 +}, Gd^{3 +}, Lu^{3 +} .$

A. $La^{3 +} > Eu^{3 +} > Gd^{3 +} > Lu^{3 +}$
B. $Lu^{3 +} > Gd^{3 +} > Eu^{3 +} > La^{3 +}$
C. $La^{3 +} > Gd^{3 +} > Eu^{3 +} > Lu^{3 +}$
D. $Eu^{3 +} > Gd^{3 +} > La^{3 +} > Lu^{3 +}$

Show correct answer & step-by-step solution

Correct answer: A — $La^{3 +} > Eu^{3 +} > Gd^{3 +} > Lu^{3 +}$

Solution

$Lanthanoid contraction refers to the steady decrease in ionic radii from La^{3 +} to Lu^{3 +} .$
$This is caused by the poor shielding effect of 4 f electrons as the nuclear charge increases.$
$The atomic numbers are La (57), Eu (63), Gd (64), and Lu (71) .$
$The ionic radii follow the order La^{3 +} > Eu^{3 +} > Gd^{3 +} > Lu^{3 +} .$
$Hence the answer is (A).$

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