In atomic structure, what is the term insulator?

Question

In atomic structure, what is the term insulator?

Answer 1

In terms of atomic structure there are no free electrons to carry charge. This means that the energy difference between the conduction bands and valences bands (band gap) is wide. For insulators like diamonds or quartz this is in the 5-8eV range, such that very few carriers have enough thermal energy to overcome the band gap. The average electron has ~q/kT energy, so at room temperature ~0.026eV.

Answer 2

It is an electrical insulator (a very poor conductor of electricity)

because there are no mobile electrons to conduct (carry) the charge.