Why is copper chloride green?

Answer 1

I assume you are referring to CuCl2, since CuCl is colorless, unstable in water, and easily oxidized.

The dissolution of CuCl2 in water gives [Cu(H2O)6]2+ in dilute solutions and various [CuClxH2O(6-x)]2-x ions, which are yellow to red, in more concentrated solutions. The combination of the two gives a green color to most CuCl2 solutions.

If you want to know why [Cu(H2O)6]2+ is blue or why the chloride complexes are red or yellow, it has to do with electronic transitions in the d electrons of the Cu ion. The ligands around the Cu split the d oribitals into different energies. Cu(II) has 9 d electrons, so the most energic orbital has only 1 electron while the other four d orbitals are filled with 2 electrons. Light of a certain wavelength will be absorbed by the Cu ion, sending an electron from a lower d orbital into the unfilled higher orbital. CuCl2 will absorb red and blue light to promote those electrons, and thus look green.

Read up on ligand field theory in any inorganic chemistry textbook if you need more information.

Answer 2

Two reasons. First, the 4d-electrons of copper are excited by visible light to upper levels. This absorbs light in the red region of the spectrum. Normally, that means that copper ions would appear blue in water solution. Indeed, aqueous copper sulfate is blue. But that is the case when four water molecules orient themselves around the copper ion. In copper chloride, four chloride ions orient themselves around the copper ion. This leads to a crystal field effect in one theory and a ligand field effect in another, wherein the energy levels of the 4d electrons are shifted, and so the wavelength of the absorbed light is altered.