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April 2001 Issue
Training: Electrons and Electricity, Part 2
By the NCTI

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

This installment builds upon last month’s introduction of orbital shells within an atom, explaining how electron behavior on specific shells creates electricity.

The valence shell

In the study of electricity, the atom’s outer shell is the most important one, regardless of its number of orbital shells. This outer shell is called the valence shell, and the electrons in it are called valence electrons. (See Figure 1.)

The electrons in each atom are arranged in such a way that the valence shell never has more than eight electrons. In Figure 2, the argon atom has eight electrons in its valence (or outer) shell, which is all the electrons any atom’s valence shell can contain. Therefore, no more electrons can be added to that shell. The potassium atom contains eight electrons in its third shell, but only one electron in its valence shell. Likewise, the chromium atom has only one valence electron, but has 13 electrons in its third shell. The main point is that each atom comprises a different element not only because of a difference in the number of protons in the nucleus, but also because of the number of electrons in each orbital shell.

The number of electrons in the valence shell also determines whether a particular element is an insulator, conductor or semiconductor. Generally, the fewer electrons a given element has in its valence shell, the more conductive that element is.

Electron energy level

Not all electrons have the same energy level, even though all electrons have the same negative charge. Electrons in the orbits nearest the atom’s nucleus contain less energy than those in the more distant orbits. Valence electrons, having an orbit farthest from the nucleus, have the greatest energy level. (See Figure 3.)

An electron that has enough energy added will overcome the positive attraction of the protons in the nucleus and move out of its orbit to the next higher (outer) orbit. Because a valence electron already is in the atom’s outermost orbit, a valence electron that has enough energy added will move out of its atom altogether. This action of electrons being freed from their atoms produces electricity.

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