>>12712765The least dense heaviest element would be the largest, this is because you can basically assume the nucleus is infinitely dense for practical purposes.
Osmium and tungsten are dense since they have very condensed electron shell properties, uranium is dense simply because it's the heaviest natural element.
So if I had to pick something non-radioactive probably iodine, xenon, or cesium depending on phase and chemistry. They have very wide shells and their lack of (relative) density reflects that. Alternatively astatine, francium, and radon. Or those super heavy elements, but they start to suffer from relativistic effects which bend the shell.
