The structure of elements/atoms is well understood based on their subatomic constituents. Naively, you might think that can you just keep combining increasingly larger numbers of electrons, protons, and neutrons to create new elements. However, the stability of an atom becomes problematic when the size of the nucleus approaches the interaction length of the strong force (i.e. the nucleus is too large for the strong force to hold it together). These elements are unstable and therefor not relevant as far as organic chemistry is concerned.

Furthermore, the formation of elements in the Universe is also a fairly well understood process. For elements lighter than Fe it generally occurs through nuclear fusion in the center of stars. For elements larger than Fe it generally occurs through the r-process and s-process. With these we can model nucleosynthesis extremely well and it gives us a very good idea of the elemental composition of the Universe. That being said, there could be some crazy unknown element out there but it would contradict almost everything know about atomic physics.