I was taught that a particle is assigned to a unique quantum state. As a specific example, two bound electrons can't have the same quantum numbers in an atom. And likewise one and only bound electron is assigned to one quantum state in an atom. Yet, I am reading several solid state books and they are saying that an electron moving in a conductive material (crystal lattice) can be described by a localized wavepacket with a group velocity and central energy... That makes sense intuitively... but wavepackets are themeselves superpositions of sinusoidal traveling wave solutions to shrodinger's equation (each with a wavevector k)... This makes it sound like one particle is assigned several wavefunctions (each with their own quantum numbers)... How can this be true???