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This question was triggered by another question on this site, about molecules with a total number of 13 electrons.

I couldn't think of any (except the boring answer of $\ce{Al}$), and the answers to the question are all about radicals, instead of 'normal' molecules. So that got me thinking: are all organic molecules with an odd total electron count radicals or radical ions? If so, is there any fundamental reason for this?

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Michiel
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    Some of them are even "worse": they are radical ions ;) On the other hand, some species with an even number of electrons aren't "normal" either: Think in carbenes or nitrenes ;) – Klaus-Dieter Warzecha Feb 20 '14 at 06:55
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    Good point! I just edited it into the question. I'm really curious whether there is any fundamental reason for the lack (absence?!) of 'normal' molecules with an odd total number of electrons – Michiel Feb 20 '14 at 06:59
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    If you pair up all single electrons, you will always get an even number (any integer * 2 is even). If you don't, you will have an unpaired electron, and that's what a radical is. –  Feb 20 '14 at 09:39
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    @Brian mmm, good point. Didn't think of that, but makes perfect sense – Michiel Feb 20 '14 at 11:44
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    Are we counting organometallics? – matt_black Feb 21 '14 at 09:46
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    @Klaus-DieterWarzecha don't forget the hydride. Smallest Lewis base I could think of. – urquiza Nov 23 '21 at 05:26

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By IUPAC definition of radical, the organic molecules with odd number of electrons must have at least one of them unpaired and are therefore radicals. The fundamental reason is the definition.

Bear in mind, that other spin states must be taken into account, as simplest example the oxygen. It exists as a triplet, so two unpaired electrons.

The situation gets different, when you do have a metal in your compound, but it is out of the scope of the question.

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