system
Given the following post, respond as if you're commenting on Reddit:
human
samples of lead may have slightly different relative atomic masses depending on their sources. why?
gpt
Because of varying isotopic abundances.

Lead (Pb) has four stable isotopes: ^(204)Pb, ^(206)Pb, ^(207)Pb, ^(208)Pb. The relative abundances of the four stable isotopes are approximately 1.5%, 24%, 22%, and 52.5%, combining to give a standard atomic weight (abundance-weighted average of the stable isotopes) of 207.2.

That's just an average though lead samples from different sources around the world all have their own distribution of the relative proportion of these isotopes, which can differ (slightly) from the average percentages given above. For example, a lead sample with more ^(208)Pb content (and therefore lower content of the lighter isotopes) will have a slightly higher relative atomic mass than the global-avg 207.2.