Atomic structure. Conceptual computer artwork of nine electrons orbiting a central nucleus. Other particles are seen around the atom. This is a classical schematic Bohr model of an atom. In quantum physics it is thought that the electrons do not actually follow a defined orbit, but instead inhabit areas of probability around the nucleus. A nucleus without any orbiting atoms is at bottom right. A lone electron is at top. At left is a proton, a positively charged subatomic particle that is made up of three quarks held together by gluons.