We did not reach this material in class but I leave it here to help you learn this subject.
Solids and liquids have densities which exceed those of gases by typically three orders of magnitude. While the liquid state is dynamic in origin as gases, the solids state is characterized by a definite static average microscopic structure. Single crystals are rare and fascinating examples of the periodic order which exists within domains of more common materials such as the steel which goes into the chairs that you are sitting on. Solid state physicists like myself use our time trying to relate microscopic properties of solids to macroscopic properties. Here we shall mostly introduce a few macroscopic materials properties which differ from one solid to another.