If an atom containg two valence electrons is struck bya moving electron, the energy of the striking electron will be divided between the two valence electrons. If the valence electrons are knocked out of orbit, they will contain only half the energy of the striking electron. This effect can also be seen in the game of pool. If a moving cue ball strikes two stationary balls at the same time, the energy of the cue ball is divided between the two stationary balls. Both stationary balls will move, but with only half of the cue ball.
Insulators are materials that resist the flow of electricity.
When the valence shell of an atom is full, the electrons are held tightly and are not given up easily. Some good examples of insulator materials are rubber, plastic, glass, and wood. The energy of the moving electron is divided so many times that it has little effect on the atom. Any atom that has seven or eight valence electrons is extremely stable and does not easily give up an electron. Semiconductors are the materials that are neither good conductors nor good insulators.