In this scenario, you can think of a reaction as terminating a molecule's life. So if there's a 50% chance that an H2O (or CO2) molecule reacts in a certain period, that could be its half-life time.

If you buy into the Big Rip, then all particles in the universe (including protons and neutrons) will eventually disintegrate

If we're talking about these kinds of scales, N2 molecules are not stable because there's a non-zero probability for the atoms to fuse into a heavier element through tunneling. And this will release more than enough energy to break the chemical bonds, of course.

Maybe you saw this story recently?: https://phys.org/news/2025-05-universe-decay-years-sooner-pr...

Also, bismuth was once thought to be the most massive "fully" stable element, but turns out does decay with a half life of 10^19 years, compared to the universe's age of ~10^10 years.

Neutrons decay into a proton/electron pair after 15 minutes when not part of a nucleus.

Protons appear to be fully stable for any practical considerations, however they might decay after 10^30 years.