Both, the comb and the hair are comprised of atoms, which are, on their part, comprised of electrons ( negatively charged particles). When Sandra combs her hair, friction between comb and hair occurs. Since the electrons in hair are more loosely connected to each other than the ones in comb, during this friction some electrons from the hair will be transferred to the comb. Since electrons are negatively charged and the comb now has excess electrons that means that the comb becomes negative and the hair becomes positive ( because it loses negative charge). This friction is responsible for charging the hair and the comb. Finally, when the comb is placed near the hair, hair will move toward the comb since the opposite charges attract.
valance electrons that reside in the outermost shell of an atom in the highest energy level. They are important to atoms because the fewer valence electrons that the atom holds, it becomes less stable.
