Transition and InnerTransition Elements - Result Question 109
####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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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.
