As far as I understand, in a primitive picture insulators are localized systems, and metals are itinerant electron systems.
What I do not get: I have, e.g., 3d itinerant magnets like:
Ni ([Ar] 4s2 3d8 or [Ar] 4s1 3d9 ), Fe ([Ar] 4s2 3d6), and Co ([Ar] 4s2 3d7).
Why does it make sense to speak about 3d electron shells, if the electrons are not localized to the atomic nucleus but they are itinerant, i.e., there are distributed over the whole crystal (it can be described by the probability function of the Bloch waves).
Confinement to atomic nucleus does not make sense to me, since (weak correlated) metals behave as a free electron gas.