Which non-synthetic element has the largest atomic mass?
The non-synthetic element that has the largest atomic mass is uranium, although it is radioactive. The atomic mass of uranium is 238.0289.
Does The element with the largest subscript in a formula have the largest percent by mass in the compound?
The naturally occurring element with the largest mass would be uranium. The largest atomic mass for an artificial element (or any element) would be Ununoctium. Neutron stars, which are neutrons densely packed together surrounded by a thin layer of degenerate matter could possibly be considered atomic nuclei, though their specific proton count would probably not be measurable. These, at perhaps 12 miles in diameter, would of course be larger yet.