[kct]

I just took an exam and I'm dying to know the answer to this question:

Pb-238(I think this was the mass number but i'm not 100& sure), has a half life of 23.8 days. Yet, it occurs naturally and we can still find traces of it today. Why?

I thought it was because it is the product of radiocative elements decaying.. Could this be possible?

[Borek]

I thought it was because it is the product of radiocative elements decaying.. Could this be possible?

Possible, although decay is not the only possible source of radioactive isotopes. Think C-14 for example. Think about all atoms being in the vicinity of radioactive one.

[Grejak]

I thought it was because it is the product of radiocative elements decaying.. Could this be possible?

Yeah, that is one of the possible reasons, especially for heavier isotopes such as 226Ra.  Another possible reason is cosmic ray interactions in the upper atmosphere.  About 99% of cosmic rays are hydrogen or helium, but the other 1% cover the range of the periodic table.  When these rays collide with the atmosphere they can cause nuclear reactions, leading the radioactive isotopes.

[jdurg]

The more likely, and probably correct, answer in this case is the decay of other radioisotopes.  Because there are other elements with isotopes that have half-lives much longer than 23.8 days, it's highly probable that at least one decay chain has that particular isotope of lead in it.

[Mitch]

There is no such thing at Pb-238.