What makes a material an insulator?

[Fig Neutron]

(This is a two part question.)

Ok, so I have been trying to find what property of a material determines if it is a good conductor or not. I'm hoping to go beyond just if it can conduct electricity or not. One of the explanations I have found is the length of the band gap. Is this right or have I missed something?

Regardless of whether this is the answer for the first question, what causes a band gap? What is in this gap or is it empty? And why can't an electron exist within this gap?

 

[anorlunda]

It is easier to study conducting metals by contrast.

The electrical and thermal conductivities of metals originate from the fact that their outer electrons are delocalized. This situation can be visualized by seeing the atomic structure of a metal as a collection of atoms embedded in a sea of highly mobile electrons. The electrical conductivity, as well as the electrons' contribution to the heat capacity and heat conductivity of metals can be calculated from the free electron model, which does not take into account the detailed structure of the ion lattice.

Filling of the electronic states in various types of materials at equilibrium. Here, height is energy while width is the density of available states for a certain energy in the material listed. The shade follows the Fermi–Dirac distribution (black = all states filled, white = no state filled). In semimetals the Fermi level EF lies inside at least one band. In insulators and semiconductors the Fermi level is inside a band gap; however, in semiconductors the bands are near enough to the Fermi level to be thermally populated with electrons or holes.

I apologize if this stuff is a bit difficult for B level, but you asked specifically about the gaps.

 

[Fig Neutron]

Thanks, I think I get it.