This lecture discusses energy bands, bonds, and the differences between metals, insulators, and semiconductors. Insulators have a large band gap of 3.5-6eV, resulting in very few charge carriers at room temperature. Semiconductors have a smaller 1eV band gap, allowing their resistivity to be controlled over many orders of magnitude. The document also briefly introduces statistical mechanics and three probability distribution functions: Maxwell-Boltzmann for distinguishable particles, Bose-Einstein for indistinguishable particles with unlimited occupancy of states, and Fermi-Dirac for indistinguishable particles with single occupancy of states.