  A toy statistical model which mimics localized-to-itinerant electron
transitions is introduced. We consider a system of two types of charge
carriers: (1) localized ones, and (2) itinerant ones. There is chemical
equilibrium between these two. The solution of the model shows that there is a
crossover, at a specific value of a local interaction energy parameter $J$, at
which carriers transfer from being itinerant to a localized/mixed valence
state. The model has a very crude analogy to the observed $\gamma\rta\alpha$
transition in metal cerium.
