Transition and InnerTransition Elements - Result Question 7
7. The lanthanide ion that would show colour is
(2019 Main, 8 April I)
(a) $Gd^{3+}$
(b) $Sm^{3+}$
(c) $La^{3+}$
(d) $Lu^{3+}$
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Answer:
Correct Answer: 7. (b)
Solution:
The lanthanide ion that would show colour is $Sm^{3+}$. Colour of a compound depends on the number of electrons in $4 f$-orbitals. Electronic configuration of given lanthanides are as follows:
$ \begin{aligned} Gd^{3+} & =4 f^{7} \\ Sm^{3+} & =4 f^{5} \\ La^{3+} & =4 f^{0} \\ Lu^{3+} & =4 f^{14} \end{aligned} $
$Gd^{3+}$ have half-filled $4 f$-orbitals.
$La^{3+}$ have no electron in $4 f$-orbitals.
$Lu^{3+}$ have fully-filled $4 f$-orbitals.
Only $Sm^{3+}$ contain $4 f^{5}$. The electrons can easily undergoes excitation. That result in a formation of colour.
BETA
