|MadSci Network: Chemistry|
I think I may have seen that documentary as well, and I admit it is slightly misleading. Uranium is an extremely dense metal (density isn't quite the same as mass, something can be quite heavy but not very dense), and is also the heaviest naturally occurring metal. If you have a copy of the periodic table (look at www.webelements.com if you don't), so can see that there are plenty of things with higher atomic numbers than 92 (Uranium's number). However, these heavier atoms are unstable on Earth. They are simply "too heavy" to exist, and so decompose into smaller elements, which is the origin of radiation. Even some isotopes (variants) of uranium are radioactive, and they break down into Uranium 235 (a particular isotope that is stable). Perhaps at the centre of stars they might be stable, but even that is not 100% known. So Uranium isn't the heaviest possible element, but it is the heaviest stable element. I hope that answers your question.
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