Eli5: Which is the most massive gas?

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What is the material with the most massive molecules that we are able to keep at the gas state? (Without chemical reactions happening and without non elastic shape deformations of the molecules). And why that one?

Edit: I mean, in a multi-molecule system, so that molecules have to withstand bumps.

In: Chemistry
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If you need, you can explain it also as I am 6 or 7:)

A single molecule of any material in a large space would be considered that material in a “gaseous” state.

Currently the largest molecule ever created is PG5, but there is no theoretical upper limit.

“Non elastic shape deformation of the molecule” means that molecule has split-ie a chemical reaction has occurred. Shaking a box or compressing the gas generally doesn’t do that to a stable gas.

In this case you need to put some constraints on the question: what is the temperature and pressure we are operating at? Is there sunlight interacting with the gas? Is the gas being stored isolated from other gases? Is the gas just running around planet earth?

This question has been asked before. Start here:

https://www.reddit.com/r/askscience/comments/oqfq3/what_is_the_largest_molecule_that_can_remain_a/c3jemmr/

My mind goes first to ultra-heavy nuclei in gaseous form. Something like uranium superheated. Each molecule has a mass equal to the atomic mass of the heavy material. If it needs to be radiologically stable, bismuth-209 is the heaviest this can get, with a molecular mass of 209. If you can have shorter half-lives, then whatever the heaviest element is in that half-life range.

Now, 209 is pretty thicc, but possibly not as huge as we can get. The first heavy ketone that came to my mind, dibutyl ketone, is already 142. I’m sure I could fuck around and find much heavier molecules that still worked, but for the sake of science, I’m going to take this a different direction. If we use the heaviest stable isotopes available for producing our dibutyl ketone, we can raise its molecular mass to 171. Getting closer.

More poking around, and I found dibenzyl ketone. It normally has a molecular mass of around 210, but with the heavier isotopes that could be increased to 241, proving that hydrocarbons win out over hot bismuth.

With even larger hydrocarbons, I’m sure you could get even more, but I’m going to leave it there.