According to the Bohr model, an atom consists of a nucleus of protons and neutrons orbited by a number of electrons. In an ordinary atom, the number of electrons equals the number of protons. If it has more or fewer electrons, it is an ion. In addition, the electrons orbit in electron shells. Each shell can only contain a certain number of electrons before new electrons must migrate to the next shell or "energy level." Starting with the third energy level, however, electrons sometimes move up a level even before the current one is full.