Molecules and Solids in Physics

Everything on this page comes from one idea, that bringing atoms close together changes the energies they are allowed to have. Two atoms bound together can rotate and vibrate, and both motions are quantized, which is why molecular spectra consist of bands rather than single lines. Bring a great many atoms together and each sharp atomic level broadens into a band of levels, and the size of the gap between bands decides whether the solid conducts, insulates, or sits in between as a semiconductor. That single distinction explains why copper carries current, why diamond does not, and why a grain of silicon with the right trace of impurity can switch a circuit. The blocks give the molecular relations, the Fermi picture of the electron gas in a metal, and the band and junction results that modern electronics is built on.