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) $\endgroup$ – orthocresol Mar 26 '17 at 12:03
  • $\begingroup$ Yes it is K2O (potassium monoxide). You can also search it on the internet. $\endgroup$ – Kartikeya Badola Mar 26 '17 at 13:03
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    $\begingroup$ Yes, well, it's more commonly known simply as potassium oxide. $\endgroup$ – orthocresol Mar 26 '17 at 13:05
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    $\begingroup$ Don't use mhchem in question title due to searchability issues. $\endgroup$ – Nilay Ghosh Mar 26 '17 at 18:08
  • $\begingroup$ Not just potassium. Heavier alkali metals form colored oxides too. So how are all the heavier alkali metal oxides colored? $\endgroup$ – Oscar Lanzi Mar 26 '17 at 20:20

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! $\endgroup$ – Kartikeya Badola Mar 27 '17 at 3:37
  • $\begingroup$ Hmm, but why does it decrease? May it be due to presence of traces of suboxides? $\endgroup$ – Mithoron Apr 9 '17 at 19:03
  • $\begingroup$ Heavier, more electropositive alkali metals impart more negative charge to oxygen, raising the valence band energy. $\endgroup$ – Oscar Lanzi Apr 9 '17 at 20:29

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