Question
Why do lanthanoids form coloured compounds?
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Answer
The colour in lanthanoid ions is due to the presence of unpaired electrons in partially filled $4f $ sub$-$shells.
Due to the absorption of radiations in the visible region there arises the excitations of the unpaired electrons from $f-$orbital of lower energy to the $f-$orbital of higher energy$-$giving $f \rightarrow f$ transitions.
The observed colour is complementary to the colour of the light absorbed.
The colour of try positive ions $(M^{3+})$ depends upon the number of unpaired electrons in $f-$orbitals. Hence the lanthanoid ions having equal number of unpaired electrons have similar colour.
The colours of $M^{3+}$ ions of the first seven lanthanoids, $La^{3+}$ to $Eu^{3+}$ are similar to those of seven elements $Lu^{3+}$ to $Tb^{3+}$ in the reverse order.
Due to the absorption of radiations in the visible region there arises the excitations of the unpaired electrons from $f-$orbital of lower energy to the $f-$orbital of higher energy$-$giving $f \rightarrow f$ transitions.
The observed colour is complementary to the colour of the light absorbed.
The colour of try positive ions $(M^{3+})$ depends upon the number of unpaired electrons in $f-$orbitals. Hence the lanthanoid ions having equal number of unpaired electrons have similar colour.
The colours of $M^{3+}$ ions of the first seven lanthanoids, $La^{3+}$ to $Eu^{3+}$ are similar to those of seven elements $Lu^{3+}$ to $Tb^{3+}$ in the reverse order.
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