Why some of the materials are insulators?
They are bad conductors ,they do not allow heat transfer.
Conductors are materials that will allow electricity to flow through them. Materials that contain 1, 2, or 3 valence electrons are good conductors. Some examples of good conductors are gold, silver, aluminum, and copper. Insulators are materials, or combinations of materials, with a high number of valence electrons (5, 6, 7, and 8). Examples of good insulators are porcelain, glass, air, and rubber.