Light elements, say, those you can find in first three rows of the periodic table, can be qualitatively described using hydrogen atom-like model. Basically, it says that properties of elements are periodic, when you go through the periodic table in a consecutive manner. But then you got heavier elements. The hydrogen atom-like approximation breaks down here, the properties are still periodic, but there are many exceptions from set of simple rules that were valid for lighter elements. In some cases even quantum-mechanical methods fail to describe heavier elements, for example gold wouldn't have gold color if not treated relativistically. One can expect that going towards extremely large Z well established techniques won't prove successful.