Hopefully...
May 2001 Issue
Training: Electrons and Electricity, Part 3
This month's installment continues a series on electrons and electricity.
The material is adapted from a lesson in NCTI's Installer Technician Course.
©NCTI
An element's electrical stability is determined primarily by the number of
electrons in its outer valence shell, which can hold no more than eight
electrons. Generally, the greater the number of electrons in the valence
shell, the more stable the atom's electrons are. The number of valence
electrons determines if an element is a conductor, insulator or
semiconductor.
Identifying conductors
Elements with a small number of valence electrons permit a lot of free
electron movement and, therefore, readily conduct electricity. Copper,
silver, gold, iron and aluminum all are good conductors. Each has only one
or two electrons in its valence shell, and such elements tend to give up
their valence electrons easily (See Figure 1). Because free electrons move
from atom to atom in these elements, electrical activity occurs readily.
Some metals conduct better than others. Even though copper, silver and gold
each have only one valence electron, their conductivities differ. Silver is
the best conductor, copper is second, and gold is third. The conductivity
levels differ because of the way each element packs together into a solid
structure. Silver atoms pack more efficiently than copper atoms and thus
have more electrons that are free to move about and interact with one
another. Copper, however, costs less than silver, so it is used more often.
Identifying insulators
Insulators have their valence shells almost filled, and their electrons tend
to be fairly stable. In fact, such atoms even try to finish filling their
valence shells by capturing additional free electrons (See Figure 2).
Without the production or movement of free electrons, electrons in atoms
near each other seldom interact, hindering electrical activity. An example
of an insulator used in cable is the foam-polyethylene dielectric between
the center conductor and outer shield of a coaxial cable. Rubber, plastic
and glass also make good insulators. Insulators also often serve as
protective electrical barriers.
Identifying semiconductors
Semiconductors are neither good conductors nor insulators. The valence shell
is responsible for this characteristic. Silicon and germanium each have four
valence electrons (See Figure 3). Therefore, these half-filled valence
shells have too many electrons to conduct well and not enough electrons to
insulate well. Transistors, diodes, and integrated circuits (ICs) all use
semiconductors.
http://www.branson.org/depts/science/envrsci/unitchem/Form_and_Name_Ionic.html
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