Are most atoms (or nuclei) unchanged since they were created? 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.
 A: 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.
