# Why is potassium monoxide (K2O) coloured?

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?

• What's potassium "monoxide", $\ce{K2O}$? (Just double-checking) – orthocresol Mar 26 '17 at 12:03
• Yes it is K2O (potassium monoxide). You can also search it on the internet. – Kartikeya Badola Mar 26 '17 at 13:03
• Yes, well, it's more commonly known simply as potassium oxide. – orthocresol Mar 26 '17 at 13:05
• Not just potassium. Heavier alkali metals form colored oxides too. So how are all the heavier alkali metal oxides colored? – Oscar Lanzi Mar 26 '17 at 20:20