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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3df206

Z values: Eu = 63, Gd = 64, Tb = 65. Which of the following species have exactly seven unpaired electrons?

A $Eu X^{2 +}$
B $Tb X^{4 +}$
C $Gd X^{3 +}$
D $Tb X^{3 +}$

Answer: A, B, C

Eu is

4 f^{7} 6 s^{2}

, so

Eu X^{2 +}

is

4 f^{7}

.

Gd X^{3 +}

is

4 f^{7}

. Tb is

4 f^{9} 6 s^{2}

, so

Tb X^{4 +}

is

4 f^{7}

but

Tb X^{3 +}

is

4 f^{8}

with 6 unpaired electrons.

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