Predict which of the following will be coloured in aqueous solution? $Ti ^{3+}, V ^{3+}, Cu ^{+},$ $Sc ^{3+}, Mn ^{2+}, Fe ^{3+}$ and $Co ^{2+} .$ Give reasons for each. 

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Only the ions that have electrons in $d$ -orbital will be coloured. The ions in which $d$ -orbital is empty will be colourless.

Element Atomic Number Ionic State Electronic configuration in ionic state
$Ti$ $22$ $T i^{3+}$ $[ Ar ]\, 3 d^{1}$
$V$ $23$ $V^{3+}$ $[ Ar ] \,3 d^{2}$
$Cu$ $29$ $Cu ^{+}$ $[ Ar ] \,3 d^{10}$
$Sc$ $21$ $Sc ^{3+}$ $[ Ar ]$
$Mn$ $25$ $Mn ^{2+}$ $[ Ar ] \,3 d^{5}$
$Fe$ $26$ $Fe ^{3+}$ $[ Ar ] \,3 d^{5}$
$Co$ $27$ $Co ^{2+}$ $[ Ar ] \,3 d^{7}$

From the above table, it can be easily observed that only $Sc ^{3+}$ has an empty $d$ -orbital. All other ions, except $Sc ^{3+}$, will be coloured in aqueous solution because of $d-d$ transitions.

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