The reason for the colour of potassium dichromate (K₂Cr₂O₇) is ligand to metal charge transfer (LMCT) transition. In Cr₂O₇^2-, Cr is in +6 oxidation state and have all the 3d-orbitals vacant. Each Cr⁶⁺ ion is surrounded tetrahedrally by four oxide ions. All oxide ions have filled 2p-orbitals. There is a transfer of an electron from the filled 2p-orbitals of O^2- to the vacant 3d-orbitals of Cr⁶⁺ ion. Since 2p-orbitals of the O^2- is ungerade and the 3d-orbitals of Cr⁶⁺ are gerade, therefore, electron transition from 2p-orbitals of the O^2- to 3d-orbitals of Cr⁶⁺ ion is Laporte allowed and there is also no change in spin multiplicity during electronic transition. Therefore, transfer of an electron is Laporte and spin allowed. Therefore, potassium dichromate, K₂Cr₂O₇ (or Cr₂O₇^2- ion) is coloured.

Related Questions

What colour is dichromate?

Answer: The dichromate (Cr₂O₇^2-) ion is orange in colour.

Why is chromate yellow in colour ?

Answer: Chromate (CrO₄^2-) is yellow in colour because of ligand to metal charge transfer (LMCT) transition. There is a transfer of electron from the 2p-orbital of O^2- (ligand) to 3d-orbital of Cr⁶⁺ ion (metal).

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