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JEE Main levelSingle correct optionDifficulty 2 of 3df183

Among the following neutral gaseous atoms (atomic numbers in brackets), the one whose ground-state configuration contains a $5 d$ electron is

A $Gd$ (64)
B $Tb$ (65)
C $Pr$ (59)
D $Eu$ (63)

Answer: A

Gd takes

[Xe] 4 f^{7} 5 d^{1} 6 s^{2}

because the half-filled

4 f^{7}

subshell is favoured. Pr is

4 f^{3} 6 s^{2}

, Eu is

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

and Tb is

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

, with no

5 d

electron.

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