Why is Potassium Oxide yellowy purple?
Why is Potassium Oxide yellowy purple - surely All group 1 compounds are white or colorless unless they bond with a colored ion.
- 1 decade agoFavourite answer
who the ***** cares??
- GeorgeSiO2Lv 71 decade ago
Hi John: I recently answered a question on the colors of M2O (M = Li, Na, K, Rb,Cs): Li2O, Na2O, pure white, K2O yellowish white, Rb2O bright yellow, Cs2O orange.  No explanation of the colors is given despite fact that the M+ and O2- ions are colorless.
What happens is that like semiconductors you get bands of MOs formed in ionic lattices. The band gap between the valence band and the conduction band in common ionic solids is large and (a) the solids do not conduct and (b) the energy gap is in the UV region and the solids are usually colorless. As you go down the PT table the band gap gets smaller (starting AOs have higher energies) and now for K2O visible light in the violet region is able to promote an electron across the band gap. Since we see the complementary color to the light being absorbed K2O appears slightly yellow (it can't be purple!). Cmpds such as CdS, CdSe, PbS, PbSe are highly colored (even PbO has red and yellow forms) because they have small band gaps and were used in paints until it was realized how poisonous these heavy metals were. Cheers, drpSource(s): N. N. Greenwood, A. Earnshaw, Chemistry of the Elements 2nd ed. p 84.