Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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24 Chapter Two
a number of free electrons gather in one location,
a charge of electricity builds up. This charge
may also be called a difference in electric
“potential”. This difference in electric potential
is more commonly known as voltage and can be
compared to a difference in pressure that makes
water flow. When this potential or pressure
causes a number of electrons to move in a single
direction, the effect is current flow. So the definition
of current is the flow of electrons. Any
atom may possess more or fewer electrons than
protons. Such an unbalanced atom would be
described as negatively (an excess of electrons)
or positively (a net deficit of electrons) charged
and known as an ion (Figure 2-5). An ion is an
atom that has gained or lost an electron. Ions try
to regain their balance of equal protons and electrons
by exchanging electrons with nearby
atoms. This is known as the flow of electric
current or electricity. For more information
about voltage and current, see the section on
“Electrical Units of Measurement” in Chapter 3
of the Shop Manual.
Valence
The concentric orbital paths, or shells, of an atom
proceed outward from the nucleus. The electrons
in the shells closest to the nucleus of the atom are
held most tightly while those in the outermost
shell are held more loosely. The simplest element,
hydrogen, has a single shell containing one electron.
The most complex atoms may have seven
shells. The maximum number of electrons that
can occupy shells one through seven are, in
sequence: 2, 8, 18, 32, 50, 72, 98. The heaviest
elements in their normal states have only the first
four shells fully occupied with electrons; the
outer three shells are only partially occupied. The
outermost shell in any atom is known as its
valence ring. The number of electrons in the
valence ring will dictate some basic characteristics
of an element.
The chemical properties of atoms are defined
by how the shells are occupied with electrons. An
atom of the element helium whose atomic number
is 2 has a full inner shell. An atom of the element
neon with an atomic number of 10 has both a full
first and second shell (2 and 8): its second shell is
its valence ring (Figure 2-6). Other more complex
atoms that have eight electrons in their outermost
shell, even though this shell might not be full, will
resemble neon in terms of their chemical inertness.
Valence represents the ability to combine.
Remember that an ion is any atom with either a
surplus or deficit of electrons. Free electrons can
rest on a surface or travel through matter (or a vacuum)
at close to the speed of light. Electrons resting
on a surface will cause it to be negatively charged.
Figure 2-4.
A balanced atom.
Copper Atom
Single
Valence
Electron
Figure 2-5.
An unbalanced atom.
Figure 2-6.
Valence ring.