[thedoc (OP)]

David asked the Naked Scientists:
   How old is an Atom (say hydrogen)?
What do you think?

[chiralSPO]

This depends on how you define an atom. If you include both nucleus and the particular electron that is currently paired with said nucleus (for a neutral hydrogen atom 1 electron and one proton) then most of the "hydrogen atoms" in the universe are only tiny fractions of a second old because the electrons are constantly moving from one atom to the next and so on at a blindingly fast rate.

If, instead, one defines the atom as only the nucleus, then most of the hydrogen atoms in the universe are as old as the universe. Sure, there are some protons that have come into being more recently, either by pair production from high energy photons, or from the decomposition of free neutrons (released either by fission or fusion processes). But a vast majority of the protons that exist now were formed within the first few minutes after the big bang, over a dozen billion years ago.

Nuclei of helium and lithium were also formed shortly after the big bang, but essentially all other nuclei were created by the fusion of these light elements in the stars. I'm not really sure what sort of age this translates to, but I think it is reasonable to say that on average, the heavier the nucleus of an atom is, the more recently it formed (for stable nuclei -- in the case of radioactive nuclei, it is probably better to consider the half-life and estimate that, for instance, there is a 99% chance that a given atom of that nucleus is younger than 7 half lives)