Question

why do lanthanides form coloured compound

Answer 1

The colour of lanthanide ions is due to the presence of partly filled f orbitals. As a result it is possible to absorb certain wavelength from the visible region of the spectrum. This results in transitions from one 4f orbital to another 4f orbital known as f-f transition.

Answer 2

Explanation:

Color of a compound is mainly attributed to the presence of unpaired electrons.

General electronic configuration of lanthanides is $4f^{1-14} 5d^{0-1} 6s2$.

As most of the lanthanide elements contain unpaired electrons so, when heat energy is provided to these elements then unpaired electrons move from lower energy level to higher energy level.

Hence, when these electrons on losing energy reach towards lower energy then excess of energy is released in the form of visible light. This visible light is responsible for imparting color to lanthanide compounds.