An atom consists of a nucleus and an atomic shell. The atomic shell is made up only of electrons which are at different energy levels. In the Bohr model, these energy levels are represented as electron orbits or shells. The atomic nucleus consists of nucleons (nucleon components). Nucleons are protons and neutrons only. Protons and neutrons have an identical mass, but neutrons are electrically neutral while each proton has an elementary electric charge. The number of protons and neutrons in the atomic nucleus is identical for many atoms and only similar for others. However, the number of protons in an atom is always identical to the number of electrons in the atomic shell, because if these numbers were not identical, then the particle would be electrically charged which would make it an ion.