Transition and InnerTransition Elements - Result Question 7

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.