Kittel begins by introducing the free electron model, which posits that the electrons in a solid can be treated as non-interacting particles moving in a periodic potential. This model is a crucial starting point for understanding the behavior of electrons in solids, as it provides a simple yet powerful framework for describing the electronic structure of metals. The free electron model is based on the Sommerfeld theory, which assumes that the electrons in a metal can be described using the Fermi-Dirac distribution. Kittel derives the key results of the free electron model, including the density of states, the Fermi energy, and the electronic specific heat.