a nucleus cant be a central atom. the nucleus is part of the atom. you are probably talking about why certain atoms are central atoms in a molecule and why some arent. the answer to that question is that the more electronegative atom will always appear as a central atom.
Assuming you are talking about an atom, which you really should have stated in your question, the nucleus (the "central part" of the atom) consists of neutrons and protons. The particles orbiting the nucleus are electrons.