Why are chromates yellow?

Question

We're doing a lab in chem on patterns of colors of Inorganic compounds and we found out that the chromates we tested are yellow. Why? The original question was "Why do crows mate with canaries?" My chem teacher thought that was just *hilarious* so after a bit of decoding I got this question (don't worry I checked and it's the right question). I don't think this makes any sense whatsoever, but maybe you have an idea of why chromates are yellow?

Thanks a bunch!

Answer 1

The short answer is that they are yellow because they absorb blue light (letting green, yellow, and red pass, and these colors together look yellow to the human eye).

The long answer is rather longer.  Chemicals absorb light because electrons in the compound can take up the energy of the photon and move to a higher energy level.  Because energy levels are quantized (limited to specific energies), not every photon can be absorbed by a compound - some do not have energies corresponding to any electronic transitions in the compound.

In chromate, the transitions corresponding to visible light energies are those in which electrons are transferred from the oxygen ligands to the chromium metal center.  IN this case, the energy of the transition corresponds to the energy of a photon in the blue part of the visible spectrum.  Such a complex is called a ligand-to-metal charge transfer complex.  Such complexes are not only colored, but are much more intensely colored than transition metal complexes with d to d transitions (which are forbidden by symmetry selection rules).