LIMITATIONS  OF  THE  BOHR MODEL

Although the Bohr atomic model was a dazzling advent that explained the atomic structure quite well, it only works with simple atoms such as hydrogen and hydrogen like atoms, those having only a single valence electron (those in the outer shell of the atom). It does not account for the energy level structures of complex atoms, which involve two or more valence electrons. Although the model allowed the prediction of transitions in the simple hydrogen atom, it did not work for a complex atom such as neon (which has six electrons in its outer shell) or even for helium (which has two electrons in its outer shell). One of the major shortcomings of this theory was the angular momentum associated with every Bohr orbit. Orbits in this model are defined by Newtonian mechanics and confined by classical Newtonian laws. Consider the ground state of hydrogen (n = 1), which, according to Bohr theory, has orbital angular momentum. This assumption is required since even at ground state the electron is orbiting, and all orbiting objects must have momentum. When quantum states for hydrogen are considered in detail, though, one can prove that the ground state of hydrogen has zero angular momentum. This is not a trivial bit of math and will be spared here, but it does highlight a potential limitation with the Bohr model. Bohr was half right, and the same results that Bohr obtained an explanation for energy levels and possible transitions may be arrived at through a different approach altogether, one that deviates further from classical physics. The new approach, quantum theory, features a wave property for electrons and all other particles. In the world of quantum theory a particle like an electron can act like a wave and be described by wave mechanics.