The electron can be either bound or unbound to the impurity
atom, but the electron can also have one of 2 spins when bound.
Since electrons move back and forth between the impurity atom and
lattice, they are in diffusive equilibrium, and there is a chemical
potential μ which is the same for all states.
(a) Calculate the grand partition function for this system, and
the probability that the electron is bound and unbound. Express
your answers in terms of...