Page 25 - Teach Yourself Electricity and Electronics

Ions 5

nucleus results in a difference in the weight, and also a difference in the density, of the
element. Thus, hydrogen containing a neutron or two in the nucleus, along with the pro-
ton, is called heavy hydrogen.
The atomic weight of an element is approximately equal to the sum of the num-
ber of protons and the number of neutrons in the nucleus. Common carbon has an
atomic weight of about 12, and is called carbon 12 or C12. But sometimes it has
an atomic weight of about 14, and is known as carbon 14 or C14.
Table 1-1 lists all the known elements in alphabetical order, with atomic numbers in
one column, and atomic weights of the most common isotopes in another column. The
standard abbreviations are also shown.

Electrons

Surrounding the nucleus of an atom are particles having opposite electric charge
from the protons. These are the electrons. Physicists arbitrarily call the electrons’
charge negative, and the protons’ charge positive. An electron has exactly the same
charge quantity as a proton, but with opposite polarity. The charge on a single elec-
tron or proton is the smallest possible electric charge. All charges, no matter how
great, are multiples of this unit charge.
One of the earliest ideas about the atom pictured the electrons embedded in the nu-
cleus, like raisins in a cake. Later, the electrons were seen as orbiting the nucleus, mak-
ing the atom like a miniature solar system with the electrons as the planets (Fig. 1-1).
Still later, this view was modified further. Today, the electrons are seen as so fast-
moving, with patterns so complex, that it is not even possible to pinpoint them at any
given instant of time. All that can be done is to say that an electron will just as likely be
inside a certain sphere as outside. These spheres are known as electron shells. Their
centers correspond to the position of the atomic nucleus. The farther away from the
nucleus the shell, the more energy the electron has (Fig. 1-2).
Electrons can move rather easily from one atom to another in some materials. In
other substances, it is difficult to get electrons to move. But in any case, it is far easier
to move electrons than it is to move protons. Electricity almost always results, in some
way, from the motion of electrons in a material.
Electrons are much lighter than protons or neutrons. In fact, compared to the nu-
cleus of an atom, the electrons weigh practically nothing.
Generally, the number of electrons in an atom is the same as the number of protons.
The negative charges therefore exactly cancel out the positive ones, and the atom is
electrically neutral. But under some conditions, there can be an excess or shortage of
electrons. High levels of radiant energy, extreme heat, or the presence of an electric field
(discussed later) can “knock” or “throw” electrons loose from atoms, upsetting the balance.

Ions

If an atom has more or less electrons than neutrons, that atom acquires an electrical
charge. A shortage of electrons results in positive charge; an excess of electrons gives a
negative charge. The element’s identity remains the same, no matter how great the ex-
cess or shortage of electrons. In the extreme case, all the electrons might be removed