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We can easily smell chlorine gas and its irritating odour, but we cannot smell oxygen gas. Do few protons and electrons make such difference to our olfactory receptors?

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Chloride (Cl2) carries 34 protons and electrons.

A common example of a relatively heavy odorless gas is CO2, carrying 22 protons and electrons.

All noble gases are odorless. The heaviest, naturally occurring noble gas is Radon, which has a proton count of 86. Ununoctium, an exotic artificial element with atomic number 118, is a noble gas too. However, with a half life of approximately 0.9 ms it's difficult to sniff out whether it's really oderless.

Sulfur hexafluoride is odorless too, with a proton count of 72.

Hence, the atomic number per se is not the determining factor reason for generating the sensation of odor.

What determines whether a compound generates the sensation of odor I am unsure of, but the above compounds mentioned are all chemically inert. Note that Ununoctium decays rapidly, it doesn't react with anything. I have not been able to verify whether inertness always means odorless, but with the above examples I am inclined to believe it holds true. However, the reverse is definitely not true, as carbon monoxide is odorless, but reactive.

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  • $\begingroup$ Your point is well noted @AliceD .. i am curious about how is the nose able to discriminate these gases... i mean there will be a well marked difference between odorless gases and gases with odor which the nose recognises. Thanks! $\endgroup$ – Ayush Bohra Nov 24 '15 at 13:04
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    $\begingroup$ Actually this idea that chemically inert substances should not produce odour may seem correct but is not in a strict sense correct. Odour perception as you know happens when the odorant molecule binds to and activates a receptor. Xenon is known to bind to NMDA receptors and cause anaesthesia. It is not strictly an odorant but inert gases my point is that inert gases can initiate/block neural response. $\endgroup$ – WYSIWYG Nov 24 '15 at 14:43
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    $\begingroup$ I think we have evolved to develop perception of odours that are indicate harmful or beneficial substances (wrt CO, I think it is one of the case of why a trait has not evolved). This is my guess that the ubiquitousness of oxygen made its perception unnecessary. Bacteria can, however, sense oxygen and swim towards it (aerotaxis). $\endgroup$ – WYSIWYG Nov 24 '15 at 14:47

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