[lkmill]

isnt the half life of most types of molecules in air far shorter than 2k years? maybe i am nitpicking, but would it not be more to correct to say we are breathing the same atoms as those in caesers last breath?

edit: itchy trigger finger, think i subconsciously wanted to be the first to comment. it is stated quite early that molecules preservation is assumed. still think it would be more correct and just as interesting to discuss atoms, not molecules.

edit 2: quick research has taught me that nitrogen gas, n2, and naturally occurring isotopes do not even have a half life. they do not radioactively decay. til.

[victorNicollet]

I have seen the similar assertion "some of the water molecules you drank today were once part of a dinosaur", which is false because water molecules do not last very long when in liquid phase (they continuously swap protons, turning into hydronium ions and back).

The O-O and N-N bonds are much stronger than H-O bonds, but there are still atmospheric processes that can break them. For instance, O2 undergoes photodissociation under ultraviolet light and recombines into O3 ozone, and N2 likely also undergoes photodissociation. And obviously, the fact that living beings breathe O2...

[BurningFrog]

Photosynthesis breaks up CO₂ and H₂O molecules to make O₂ and C₆H₁₂O₆ (glucose).

I don't know how often the average water CO₂/H₂O molecule gets dismantled this way, but there can't be many left since 44 BC.

[satvikpendem]

People should instead say atoms, not molecules. Or maybe even say quarks.

[pvg]

What's the 'half-life' you're thinking of? Your basic gas molecules will last a lot longer than 2k years short of being involved in some reaction or another. And a lot of these reactions aren't that easy in atmospheric conditions- e.g. pulling nitrogen out of the atmosphere https://en.wikipedia.org/wiki/Nitrogen_cycle

[oatsandsugar]

I don't think so — Nitrogen, the most common part of air, is stable in its most common isotope

[scheme271]

That's true for most timescales, however plants and other organisms fix nitrogen in the atmosphere into biologically useful molecules so nitrogen gets cycled in and out of the atmosphere. Similar things apply for carbon dioxide and oxygen.

[lkmill]

indeed it seems so, i thought all atoms (except hydrogen) had some kind of decay. i thought so called stable atoms still had half-lives of 10^{very large number} years.

[adonovan]

> would it not be more to correct to say we are breathing the same atoms as those in caesers last breath?

You may be right, but according to quantum mechanics, you can't really meaningfully talk about the "same" atoms, or any particles, because they don't have identities. There was a particle here, now there's a particle there, but we can't say exactly where it was at all the times in between, and it may not have been at any particular place: its amplitudes may have passed through two doors at once.