The electrical properties of various materials are determined by the number of electrons in the outer ring of their atoms.
• CONDUCTORS - Materials with 1 to 3 electrons in the atom's outer ring make good conductors. The electrons are held loosely, there's room for more, and a low EMF will cause a flow of free electrons.
• INSULATORS - Materials with 5 to 8 electrons in the atom's outer ring are insulators. The electrons are held tightly, the ring's fairly full, and a very high EMF is needed to cause any electron flow at all. Such materials include glass, rubber, and certain plastics.
• SEMICONDUCTORS - Materials with exactly 4 electrons in the atom's outer ring are called semiconductors. They are neither good conductors, nor good insulators. Such materials include carbon, germanium, and silicon.
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