While atoms are forming bonds, the electrons are moving closer and closer together, until the Pauli exclusion principle must be considered. To avoid the violation of the exclusion principle that would result when electrons with the same energy, etc. occupied the same localized region of space, the electron energy levels shift somewhat. To understand how this occurs, imagine taking six sheets of posterboard and laying them side-by-side flat on the floor, as shown in the image below. Initially, all six sheets have the same potential energy because they are at the same height from the floor. As you shove those six sheets into each other, something has to give. Click on the button to see an animation of what happens text. In this analogy, the "something" that gives is the height of five of the posters. The thin edges being pushed together eventually slide over each other, and you end up with six posterboards on top of each other. All initially had the same height, but now they are at six different heights. The single energy level has split into six different energies.
If you were to "push" six silicon atoms together, the electron energy levels would be altered just like the poster board energy levels were in our analogy. If six atoms are close together, then each energy level of the original atoms splits into six slightly different energies. The enormous number of atoms in solids results in an enormous number of energies available which are so close to each other that they appear continuous. We say that the valence electrons in the solid occupy a "band" of energies. Other energy shells also morph into bands in solids. Like the original energy shells, these bands are separated by ranges of energies that electrons cannot have. These forbidden regions of the energy diagram are called "band gaps." The animation below depicts one way of visualizing band formation, using energy diagrams. Follow the directions in the animation, and click through to the end.
Images reproduced (with permission) from the Energy
Band Creator simulation of the Visual
Quantum Mechanics project.
Click here to bring up a window with instructions for using the simulation on your own to step through the process of band formation.
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Copyright © 2003 Doris Jeanne Wagner and Rensselaer Polytechnic Institute. All Rights Reserved.