370770_ch_04.qxd 1/6/03 8:05 AM Page 93 mac45 Mac 45:1st Shift:
CHEMISTRY IN FOCUS
Glowing Tubes for Signs, Television Sets, and Computers
J. J. Thomson discovered that atoms contain electrons by using a device called a cathode ray tube
(often abbreviated CRT today). When he did these
experiments, he could not have imagined that
he was making television sets and computer monitors possible. A cathode ray tube is a sealed glass
tube that contains a gas and has separated metal
plates connected to external wires (Figure 4.7).
When a source of electrical energy is applied to the
metal plates, a glowing beam is produced (Figure
4.8). Thomson became convinced that the glowing
gas was caused by a stream of negatively charged
particles coming from the metal plate. In addition,
because Thomson always got the same kind of
Source of
electrical
potential
Stream of negative
particles (electrons)
(–)
(+)
Metal plate
Gas-filled
glass tube
Metal plate
Figure 4.7
Schematic of a cathode ray tube. A stream of electrons
passes between the electrodes. The fast-moving particles excite the gas in the tube, causing a glow between the plates.
If the atom were expanded to
the size of a huge stadium, the
nucleus would be only about as
big as a fly at the center.
Figure 4.8
A CRT being used
to display computer graphics.
negative particles no matter what metal he used,
he concluded that all types of atoms must contain
these same negative particles (we now call them
electrons).
Thomson’s cathode ray tube has many modern
applications. For example, “neon” signs consist of
small-diameter cathode ray tubes containing different kinds of gases to produce various colors. For
example, if the gas in the tube is neon, the tube
glows with a red–orange color; if argon is present,
a blue glow appears. The presence of krypton gives
an intense white light.
A television picture tube or computer monitor
is also fundamentally a cathode ray tube. In this
case the electrons are directed onto a screen containing chemical compounds that glow when
struck by fast-moving electrons. The use of various
compounds that emit different colors when they
are struck by the electrons makes color pictures
possible on the screens of these CRTs.
these results could be explained only in terms of a nuclear atom—an
atom with a dense center of positive charge (the nucleus) around which
tiny electrons moved in a space that was otherwise empty.
He concluded that the nucleus must have a positive charge to balance the negative charge of the electrons and that it must be small and
dense. What was it made of? By 1919 Rutherford concluded that the nucleus of an atom contained what he called protons. A proton has the
same magnitude (size) of charge as the electron, but its charge is positive.
We say that the proton has a charge of 1 and the electron a charge
of 1.
Rutherford reasoned that the hydrogen atom has a single proton at
its center and one electron moving through space at a relatively large
93