How much does Thorium weigh? This might seem an easy question to answer but when an atom has isotopes and is slightly radioactive it gets complicated. If you are not into details, this is not for you.
The standard atomic weight is defined as the mean value of atomic weights of a number of "normal samples" of the element in question. A normal sample is any reasonably possible source of the element or its compounds in commerce for industry and science that has not been subject to significant modification of isotopic composition within a geologically brief period.
The standard atomic weights of thorium have now been changed from 232.038 06(2) to 232.0377(4). The changes in weight have occurred due to a greater understanding of the prevalence of different versions of the isotopes. Thorium has also been reclassified from a mono- to a bi-isotopic element owing to the significant abundance of the thorium-230 isotope in deep seawaters.
At the same time the Commission on Isotopic Abundances and Atomic Weights (www.ciaaw.org) recommended changes to the standard atomic weights of 19 chemical elements:
Updated Periodic Table with revised values for atomic weights
International Union of Pure and Applied Chemistry
US Geological Survey