1.

Why do lanthanoids form coloured compounds?

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 forbital of higher energy-giving f → f transitions.
  • The observed colour is complementary to the colour of the light absorbed.
  • The colour of try positive ions (M3+) 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 M3+ions of the first seven lanthanoids, La3+ to Eu3+ are similar to those of seven elements Lu to Tb3+ in the reverse order. 


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