Bohr Theory

 Bohr Theory was the foundation which started many people to look at the behavior of electrons in new ways. The theory has some clearly defined problems, ideas that we now believe were incorrect. Those problems have been modified over the years so that the final description of the behavior of the electron is now more accurate. Ultimately, the Bohr Theory led scientists, specifically Erwin Schrodinger, to the Modern Theory of Atomic Structure. The primary limitation to Bohr Theory is that it was limited to a description of a one electron system, namely Hydrogen. The description of multiple electron systems is much more complex, and was only adequately handled by the Modern Theory of Atomic Structure.

 Basically, the Bohr Theory consists of six parts. Each part contributes a component to the, overall, understanding of electron behavior. Listed, here, are those six ideas.

  1. The electron travels in a circular path around the nucleus. This path is called an orbit.
  2. At normal living conditions, room temperature, the electron resides in the orbit which is closest to the nucleus. This is the position of lowest energy content for the electron, and is referred to as the Ground State. (This statement implies that there will be more that one orbit available to an electron.)
  3. As long as the electron remains in a specific orbit, no energy is gained or lost by the system.
  4. If energy is added to an electron, the electron will move to a new orbit. This orbit will be farther from the nucleus, and is a position of higher energy content. This new position is known as an excited state.
  5. When an electron moves from one orbit to another orbit, it does so without ever passing through the space between the orbits. In other words, the electron is only allowed to exist at very specific distances from the nucleus, or positions of very specific energy content. (This idea is much like climbing a ladder. The foot is only allowed to be placed in very specific locations.) This idea is known as a quantum jump, a transition in which the electron gains or loses a very specific amount of energy.
  6. When an electron is in an excited state, it will always drop down to a lower energy state, ultimately returning to ground state. Each electron transition to a lower energy state will be accompanied by the simultaneous release of energy. This energy is released as electromagnetic radiation. The energy of the released radiation will correspond to the difference in energy content between the two levels.

 Part Five off the Bohr Theory is, perhaps, the most controversial item. It says that an electron is restricted to having certain specific quantities of energy. The electron will never be allowed to have energy in between the allowed values. This is referred to as the quantization of energy. The idea was first expressed by Max Planck. This piece of information, when given to Bohr, suddenly made the ideas that he expressed much more meaningful. In essence, it now becomes clear why an atom will only release specific colors of light, or specific wavelengths of electromagnetic radiation. Without the Planck contribution, the Bohr atom would release all colors of light.


 So, what are the problems with the Bohr Theory? There are three problems which are immediately identified.

  1. The theory only works for a one electron system. What happens when an atom has more than one electron?
  2. The theory violates the Heisenberg Uncertainty Principle. Bohr Theory makes the behavior of the electron entirely to predictable. Bohr claims it is possible to know exactly where an electron is and what it is doing. The Heisenberg Uncertainty Principle says that is not possible.
  3. The Bohr Theory will based on trying to explain four visible colors in the hydrogen atomic spectrum. He worked with a red line, blue-green line, blue line, and violet line. With improved instrumentation, it is now known that the red line is actually two red lines. These lines are extremely close together, and are referred to as a doublet. The instruments that were available to Bohr were not sophisticated enough to distinguish the two red lines. To him they looked as if they were one wide red line.

 

 Questions and comments should be sent to :   kdrews@bcpl.net  

Updated November 10 ,2000