# Why is O2 (Oxygen) colorless and odorless while O3 (Ozone) is a pale blue gas with a distinctively pungent smell?

Why does adding one more oxygen molecule to the bond make it smelly and have color?!

The real answer for why ozone smells the way it does has little to do with chemistry but perhaps a lot to do with evolution.

The specific reason why it is coloured far more intensely than oxygen is explained well in the previous answer but it is worth adding a simple point. There is very little reason to think that small changes in structure make only small changes to chemical properties. Hydrogen peroxide is only a single oxygen different from water, but is a strong and dangerous oxidising agent; chlorine gas is just two electrons away from two chloride ions but one is a poison gas and the other a large part of table salt.

As for the smell it would be very inconvenient if we smelled oxygen. The level of oxygen is essentially constant and we need it to breathe. Smelling it would be useless and would get in the way of the ability to smell other things. And the brain would filter out a constant signal anyway (as it does with plenty of other stimuli). Ozone is uncommon and harmful. We might not have evolved specifically to smell it, but we have generic smell senses for unusual things many of which are also harmful. It is useful to be able to smell unusual, rare substances as it helps either avoid them or find them depending on whether they are dangerous or not.

• Wow! Thanks for elobarating on the other person's answer, lots of smart people on these forums, will defintely come back here if I got any other chemistry related questions. I hope to dig deeper into chemistry in the Fall :)
– Alex
Jul 14 '16 at 19:49

Our perception of colour and smell involve different chemistry so there is no reason why they should be related. For example, what ever the chemistry of smell is (and its not certain what this is) it clearly does not involving photons and subsequent isomerisation of a rhodopsin.

Ozone is slightly coloured as it has two low lying electronically excited states (Chappius bands 550 to 600 nm) and a second state (at approx 700 to 1000 nm) both of whose energy gaps from the ground state correspond to a wavelength in the visible spectrum. The combined absorption is rather broad, weak and peaks at approx 600 nm which in the red part of the spectrum, thus ozone appears blue. Oxygen also has two low lying electronic states at around 700 nm and 12000 nm but are very weak and the longer wavelength transition is beyond the range of our eyes to detect it (liquid oxygen appears blue as does liquid ozone). Both O$_2$ and O$_3$ have electronic states in the ultra violet at a wavelength beyond the range of our eyes.

( I have added a moderately detailed description as to why high pressure gas and liquid oxygen molecules are coloured. see Does O2 have a color in the gas phase )

• Incredible answer, you really went into detail why it shows color. I'm glad the others explained why it's smells. I really learned from this one thread. This site is truly incredible!
– Alex
Jul 15 '16 at 3:58

Smell: this is going to be lie. Simple answer: ozone is way more reactive (more poisonous than Cl2 that was used in WWI), so it burns your nose, nakes reactive chemicals that you smell. Correct answer. Brain receives the map of smells. There are no receptors (in the usual understanding) in the nose. That is why people change smell perception as they age.

Color: A simple answer: ozone has a system of conjugated bonds and a resonance structure. The longer the resonance structure the more "red-shifted" the absorption will be. Ozone has its absorption tail reaching visible blue light. Dioxygen doesn't. Long answer. Well, what is the color of water? Is it colorless or blue? Why oxygen gas is transparent, but liquid oxygen is blueish? Why skies are blue? These effects are quantitative and are not derivable apriori (from first principles).

• I'd like to add, is there a such thing as liquid Ozone? If there can be liquid oxygen??
– Alex
Jul 15 '16 at 4:10
• @Alex It exists. Wiki chemistry has more about it. en.wikipedia.org/wiki/Ozone Jul 15 '16 at 14:38