13.1. Electron Shells Revisited
In Chapter 1, we introduced the idea of electron shells around the nucleus of an atom. Atomic structures like copper, silver, and gold have a single electron in their outer (valence) shell and are good electrical conductors. Atomic structures like sulfur, whose outer shells are filled, are electrical insulators.1 Figure 13-1 shows a comparison of the atomic structures of copper and silicon. Silicon has four electrons in its outer shell.
1. This simplifies the distinction between conductors and insulators a little, but not much. For a good discussion of the difference, see http://en.wikibooks.org/wiki/Semiconductor_Electronics/Types_of_Materials ...