Why do lanthanoids form coloured compounds?

Explanation

(1) The colour in lanthanoid ions is due to the presesnce of unpaired
electrons in partially filled 4 f sub-shells.
(2) 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 rarr f` transitions.
(3) The observed colour is complementary to the colour of the light
absorbed.
(4) The colour of tripositive 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.
(5) 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.