Electricity flows on an atomic level from electrons traveling between atoms in a substance. These valence, or free, electrons vary between substances, causing them to exhibit varied electrical properties. Those substances that carry electrical current well are known as conductors, while substances that inhibit this travel are known as insulators.
Materials that possess atoms with freely moving electrons are utilized as conductors in every industry and field requiring electricity. Many metals have good conductive properties, including copper, aluminum, steel, lead, gold and silver. Other metals, such as iron, can carry electrical currents, but not as efficiently.
Some materials possess the properties of both conductors and insulators, such as silicone and germanium. These substances are the heart of modern electronics and feature four valence electrons, providing the switching abilities required by transistors, integrated chips and computer processors. Other semiconductors include solar photo-voltaic cells, light-emitting diodes (LEDs) and rectifiers.
Most the substances found in nature fall in the category of insulators. These substances are usually a combination of more pure insulators, such as wood, clay and rocks. Straw, cloth and resin have all been used as electrical insulators in the past, and porcelain has been traditionally used with high- and low-power lines in early construction.
Substances which have seven or eight valence electrons are known as insulators. These inhibit the flow of electricity on the atomic level, and are used to separate and isolate electrical signals. Examples of traditional insulators include rubber, plastics and glass. Synthetic insulators have also been developed, including nylon and paper composites.