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Most atoms we experience (on this planet, for simplicity) were created very long time ago in supernova explosions and the like.

The light elements (eg helium made in the Sun) and some heavy ones (nuclear reaction products) are sometimes newer. But most of the other ones are remnants of long-gone stars. Gone through countless reactions but still the same.

My question is: these atoms that are here for such a long time, can we think about them as unchanged?

Perhaps electrons are physically exchanged when creating bonds. But I wonder if nuclei of these elements ever change physically? Can we perceive atom nuclei as the very same matter that was produced by the stars eons ago?

I do realize it's hard to talk about matter on that scale. All atomic particles may better be described by quantum wave functions. But since they are the building blocks of the macroscopic matter that we know the question whether they are physically the same stuff seems valid.

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  • $\begingroup$ What kind of change are you imagining? If you are only talking about those properties that determine how the atoms interact with other atoms to form chemical substances, then yeah. For stable atoms, those properties, as far as we know, never change. $\endgroup$ – Solomon Slow Nov 15 '19 at 15:10
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Short answer: Yes, mostly, with some exceptions.

Long answer:

Nuclei can only be changed by nuclear processes. So the everyday chemical transformations that go on in our bodies and in the world around us do not affect the nuclei of the atoms involved.

However, some nuclei are naturally unstable and will spontaneously decay or transform themselves into other nuclei at a steady rate. These unstable nuclei may have been formed in supernova explosions before the birth of the solar system, but are still around because they decay very slowly - an example is thorium-232 with a half life of over $14$ billion years. Or they may be "daughter products" created by the decay of other unstable nuclei. Or they may be created by the irradiation of stable nuclei by neutrons produced in an atomic reactor or an atomic bomb or by cosmic rays (which are responsible for carbon-14 production in the atmosphere).

And finally, stable nuclei can be transformed into other nuclei under conditions of extremely high temperature and pressure. This is the process of nuclear fusion, which happens all the time in the core of the sun. On Earth this only happens for very short periods in fusion reactors.

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    $\begingroup$ In your list of processes, you omit cosmic rays. This is why carbon 14 dating works. $\endgroup$ – user4552 Nov 15 '19 at 16:25
  • $\begingroup$ @BenCrowell Good point. I have added carbon-14 production to my answer. $\endgroup$ – gandalf61 Nov 15 '19 at 16:35
  • $\begingroup$ Thanks for the thoughtful comment. Also the examples of how some heavy elements can be created get isotopes are interesting, notably the Carbon-14 production. My question is rather about the nature of the matter made of the non-changed stable / or extremely slowly decaying elements (probably the overwhelming majority). Could we say that matter is really the same stuff that was ever created in supernovae etc. all those eons ago? $\endgroup$ – Vacilando Nov 15 '19 at 22:28

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