Flexi Says: in 1911, Ernest Rutherford discovered the nucleus of the atom and proposed the planetary model. The planetary model said that the electrons moved around the nucleus like planets orbiting the sun, at random. In 1913, Bohr, found that that electron orbitals are actually located at fixed distances from the nucleus. His model demonstrated the existence of energy levels located at these distances. Physicists in the 1920s found that electrons do not actually travel in fixed paths, but instead only have a certain chance (calculated by mathematical formulas) of being in a particular place. Part of this discovery was explained by the discovery of the existence of wave-like properties of electrons and particles of matter. Modern atomic theory says that electrons only exist where a wave (orbital) is stable, which means they cannot exist outside of (spaces between) of the orbitals.