Question
Explain, why $Ce ^{3+}$ ion is colourless.

Answer

The electronic configuration of $Ce^{3+}$ is, $[Xe] 4f^7$
Even though there is one unpaired electron in $4f$ sub$-$shell, the $f \rightarrow f$ transition involves very low energy. Hence, $Ce^{3+}$ ion does not absorb radiation in the visible region.
Therefore $Ce^{3+}$ ion is colourless.

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