Potassium monoxide is rarely formed and is a pale yellow solid. I don't understand how is this compound coloured because in oxide anion electron cannot excite in the next orbital due to huge energy difference. Also there are no empty molecular orbitals like there are in peroxide and superoxide anion. The colour cannot come from $\ce{K+}$ cation because electron cannot be excited. So how is this compound coloured?

  • $\begingroup$ What's potassium "monoxide", $\ce{K2O}$? (Just double-checking) $\endgroup$ Mar 26, 2017 at 12:03
  • $\begingroup$ Yes it is K2O (potassium monoxide). You can also search it on the internet. $\endgroup$ Mar 26, 2017 at 13:03
  • 3
    $\begingroup$ Yes, well, it's more commonly known simply as potassium oxide. $\endgroup$ Mar 26, 2017 at 13:05
  • $\begingroup$ Not just potassium. Heavier alkali metals form colored oxides too. So how are all the heavier alkali metal oxides colored? $\endgroup$ Mar 26, 2017 at 20:20

1 Answer 1


Probably as good an explanation as any can be found here.


Basically, the electronic structure of an ionic compound like potassium oxide has "valence bands", which are based primarily on the anions (oxide ions), and empty "conduction bands" based on the cations (potassium ions). Typically, to excite electrons from the valence band to the conduction band you need to absorb ultraviolet photons and thus you get no color. But potassium oxide has just a small enough energy gap between valence and conduction bands to allow some electrons to be excited with violet light. That absorption gives the yellow color.

If we go further down the alkali metal oxides the band gap gets smaller, and more visible light with longer blue/green wavelengths may be absorbed. Then the color becomes stronger and more reddish.

  • $\begingroup$ I was thinking that the answer is related to the molecular orbitals in the lattice. Thank you so much for the answer! $\endgroup$ Mar 27, 2017 at 3:37
  • $\begingroup$ Hmm, but why does it decrease? May it be due to presence of traces of suboxides? $\endgroup$
    – Mithoron
    Apr 9, 2017 at 19:03
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    $\begingroup$ Heavier, more electropositive alkali metals impart more negative charge to oxygen, raising the valence band energy. $\endgroup$ Apr 9, 2017 at 20:29

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