The ease with which an electron moves from one atom to another determines the conductivity of a material. Conductance is the ability of a material to carry electric current. To produce current flow, electrons must move from atom to atom. Materials that easily permit this flow of electrons from atom to atom are classified as conductors.
Some examples of good conductors are copper, aluminum, gold, silver, iron, and platinum. Materials that do not easily permit, or perhaps prevent, a flow of electrons are classified as insulators. Some examples of good insulators are glass, mica, rubber, and plastic. Sometimes it can be more difficult to determine. For instance, pure (distilled) water is a good insulator, whereas tap water (because of its mineral load) is a conductor of electricity.
• A conductor is generally a metallic element that contains fewer than four electrons in its outer shell or valence. The electrons in atoms of elements with one to three electrons in the valence shell are not so tightly bound by the nucleus.
• An insulator is a nonmetallic substance that contains five or more electrons in its outer shell or valence. When an atom has five or more electrons in the valence they are more tightly bound to the nucleus, meaning that they are less likely to be given up.
• Semiconductors are a group of materials that cannot be classified either as conductors or insulators. They have exactly four electrons in their outer shell. Silicon is an example of a semiconductor.
Any material that can conduct electricity, even when in an electrically neutral state, contains vast numbers of moving electrons that move from atom to atom at random. When a battery is placed at either end of a conductor such as copper wire and a complete circuit is formed, electrons are pumped from the more negative terminal to the more positive. The battery provides the charge differential or potential difference, and the conductive wire provides a path for the flow of electrons. The transfer of electrons continues until either the charge differential ceases to exist or the circuit path is opened. The number of electrons does not change. In order to have a continuous transfer of charge (that is, have a continuous flow of current) between two points in an electrical circuit, it is necessary to produce a new supply of electrons at a negative point in the circuit as fast as this supply is consumed at a positive point in the circuit. A variety of methods of achieving this exist, and they are covered in a later section in this chapter under the heading “Sources of Electricity”.