system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is lead the best element for stopping radiation? It's a big atom, I would assume that small atoms arranged in a highly compact lattice would be best.
gpt
Lead has the unique quality of possessing both a very high molecular weight and a very dense molecular structure. Those qualities make it appealing as a radiation shield material. 

It's more commonly used over even denser elements because it's extremely cheap compared to elements like tungsten.