I've often wondered, but have never found a solid explanation for, why the chemical elements are found together in nature? Why aren't atoms all in a somewhat homogenous mix? Why are X atoms found with other X atoms? And why are isotopes found together, such as Uranium 235 and the far more common Uranium 238? They aren't even formed in the same decay chains. Does it simply have to do with atomic mass, or is there some other reason?