# Why is nitrogen dioxide (NO2) brown? [closed]

I don't understand why nitrogen dioxide $$\ce{NO2}$$ is coloured brown when all of the other nitrogen oxide species and even nitrate are otherwise colorless. I understand it is due to absorption of light when it frequency matches a vibration frequency but what makes $$\ce{NO2}$$ special?

## closed as too broad by NotEvans., Jannis Andreska, Mithoron, Jon Custer, TyberiusSep 16 '17 at 23:13

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