Opposing the attraction between the two particles, and
thus preventing the electron from moving into the proton,
is the centrifugal force on the electron caused by its
a. When there are more than two electrons in an
circular path around the proton. This is the same sort of
atom, they will move about the nucleus in different size
balance achieved if a ball tied to a string was whirled in a
orbits. These orbits are referred to as shells. The
circle in the air. The centrifugal force exerted tries to
innermost shells of the atom contain electrons that are
move the ball out of its circular path, and is balanced by
not easily freed and are referred to as bound electrons.
the string (the attractive force). If the string should
The outermost shell will contain what is referred to as
break, the centrifugal force would cause the ball to fly
free electrons. These free electrons differ from bound
away. Actually, this is what happens at times with atoms.
electrons in that they can be moved readily from their
The attractive force between the electron and proton
sometimes is not great enough to hold the electron in its
circular path, and the electron gets away.
b. If a point that has an excess of electrons
(negative) is connected to a point that has a shortage of
e. A slightly more complex atom is shown in figure
electrons (positive), a flow of electrons (electrical current)
is an atom
of helium. Notice that there are
will flow through the connector (conductor) until an equal
now two protons in the center and that two electrons are
electric charge exists between the two points.
circling around the center.
Because there is an
additional proton in the center, or nucleus, of the atom,
an electron must be added so as to keep the atom in
charge of electricity is formed when numerous electrons
electrical balance. Notice also that there are two
break free of their atoms and gather in one area. When
additional particles in the nucleus; these are called neu-
the electrons begin to move in one direction (as along a
trons. Neutrons are necessary in order to overcome the
wire, for example), the effect is a flow of electricity or an
tendency of the two protons to move apart from each
other. For, just as unlike electrical charges attract, so do
batteries could be called electron pumps, because they
like electrical charges repel. Electrons repel electrons.
remove electrons from one part of an electric circuit and
Protons repel protons, except when neutrons are
concentrate them in another part of the circuit. For
present. Though neutrons have no electrical charge,
example, a generator takes electrons away from the
they do have the ability to cancel out the repelling forces
positive terminal and concentrates them at the negative
between protons in an atomic nucleus and thus hold the
terminal. Because the electrons repel each other (like
electrical charges repel), the electrons push out through
the circuit and flow to the positive terminal (unlike
f. A still more complex atom is shown in figure 1-1.
electrical charges attract). Thus, we can see that an
This is an atom of lithium, a light, soft metal. Note that a
electric current is actually a flow of electrons from
third proton has been added to the nucleus and that a
negative to positive.
third electron is now circling around the nucleus. There
also are two additional neutrons in the nucleus; these are
This is just the reverse of the old idea of current flow.
needed to hold the three protons together. The atoms of
Before scientists understood what electric current was,
other elements can be seen in a similar manner. As the
they assumed that the current flowed from positive to
atomic scale Increases in complexity, protons and
negative. However, their studies showed that this was
neutrons are added one by one to the nucleus, and
wrong, because they learned that the current is electron
electrons to the outer circles. After lithium comes
movement from negative (concentration of electrons) to
beryllium with four protons and five neutrons, boron with
positive (lack of electrons).
five protons and five neutrons, carbon with six and six,
nitrogen with seven and seven, oxygen with eight and
eight, and so on. In each of these, there are normally the
same number of electrons circling the nucleus as there
a. General. Any material that will allow electric
are protons in the nucleus.
current to flow through it is an electrical
Change 1 11-2