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JEE Main levelSingle correct optionDifficulty 2 of 3df185

Which of the following tripositive ions has the largest number of unpaired electrons?

A $Ce X^{3 +}$
B $Gd X^{3 +}$
C $Dy X^{3 +}$
D $Eu X^{3 +}$

Answer: B

Gd

^{3 +}

is

4 f^{7}

(7 unpaired),

Eu X^{3 +}

is

4 f^{6}

(6),

Dy X^{3 +}

is

4 f^{9}

(5) and

Ce X^{3 +}

is

4 f^{1}

(1). The half-filled

4 f^{7}

ion has the maximum.

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