Overview of the electronic structure of the different fundamental classes of materials. Atoms have discrete energy levels for each electronic state. Electronic transitions by eg. optical excitation can change the state of the atom. Molecules can also have discrete energy levels, but the more complex structure also gives a much more complex diagram of electronic states. In addition, the molecules can rotate and vibrate which modulates the observed energy levels. Insulators can be seen as a condensed phase of molecules with little electronic connection between neighboring molecules for conducting a current. Only when excitation is made with an energy above the several eV bandgap will conduction be possible. Semiconductors have a more narrow bandgap and even at room temperature a few conduction electrons will be excited into the conductance band. Doped Semiconductors have higher electrical conductance because added dopants provide conduction electrons. Metals can be considered as ionized metal atoms in a sea of free electrons, giving a high conductivity and high reflectivity of light (as long as it is not too high in frequency)