A material with many free electrons allows easy transfer of electrical energy and is therefore called a conductor. If we send an energetic electron into a conductor, it will impact a free electron, knocking it down the line until it hits another free electron. This sets up a chain reaction of impacts that conducts the electrical energy through the material.

A material that contains these types of atoms has very few, if any, free electrons and does not transfer electrical energy well, if at all. This type of material is called an insulator.

If we send an energetic electron into an insulator, it effectively bounces off the atoms, unable to transfer its energy to the tightly bound electrons. It will keep bouncing around until it either frees another electron or until it simply runs out of energy.