Electrons in Solids

Electrons in Solids - 20

Atoms generally bond together so that energy levels are filled. A silicon atom needs to find four other electrons to "borrow" (ionic bonding) or to "share" (covalent bonding). Those additional four electrons bring the number of valence electrons to the desired eight that will fill silicon's valence shell. In the silicon crystals that form the backbone of the electronics industry, each silicon atom forms covalent bonds with four other silicon atoms, sharing one of its electrons (and receiving a shared electron in return) from each of the four neighbors. Such bonding is best illustrated using a lattice diagram as shown to the right below. The red circles represent the positive core of silicon atoms. The atomic core consists of the nucleus (which contains 14 protons and 14 neutrons in the most common isotope of silicon) and the non-valence electrons (10 of them in silicon). We can let the circles include the non-valence electrons since those electrons in filled shells do not really participate in the chemical bonding process. The lines extending from each circle represent the valence electrons, 4 from each silicon atom. For individual atoms of silicon, these valence electrons do not fill a shell. The green color of these lines reminds us of the partially full shell.

Click on the image to run the animation. As the atoms approach each other, bonds form. The valence electrons are shared between atoms, and each nucleus finds itself surrounded by a full complement of eight electrons. Each atom now has a filled valence shell, as indicated by the blue color of the lines in the image. These interconnecting bonds create a crystalline structure that gives silicon many of its useful properties. Of course, the silicon devices used in information technology contain many more atoms than any illustration can hope to portray. And the atoms form 3-dimensional crystals rather than the 2-dimensional form shown here.

What happens to the energy levels of atoms when the atoms bond in a solid?
Go to the next page to find out!