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C04_016A
Electrons
Matter containing
evenly distributed
positive charge
Figure 9
J.J. Thomson’s plum pudding model of the atom
states that the atom is a uniform, positively charged
sphere containing electrons.
The plum pudding model
The existence of the electron and the knowledge of some of its properties raised some
interesting new questions about the nature of atoms. It was known that matter is
neutral—it has no electric charge. You know that matter is neutral from everyday experi-
ence: you do not receive an electric shock (except under certain conditions) when you
touch an object. If electrons are part of all matter and they possess a negative charge,
how can all matter be neutral? Also, if the mass of an electron is so small, what accounts
for the rest of the mass in a typical atom?
In an attempt to answer these questions, J. J. Thomson proposed a model of the atom
that became known as the plum pudding model. As you can see in
Figure 9
, Thomson’s
model consisted of a spherically shaped atom composed of a uniformly distributed
positive charge in which the individual negatively charged electrons resided. However,
as you are about to read, Thomson’s plum pudding model of the atom did not last for
very long.
The Nucleus
In 1911, Ernest Rutherford (1871–1937) began to study how positively charged alpha
particles (radioactive particles you will read more about later in this module) interacted
with solid matter. With a small group of scientists, Rutherford conducted an experiment
to see if alpha particles would be deflected as they passed through a thin gold foil. If
they were deflected, by how much would they deflect?
Rutherford’s experiment
In the experiment, a narrow beam of alpha particles was aimed at a thin sheet of gold
foil. A zinc-sulfide coated screen surrounding the gold foil produced a flash of light
when struck by an alpha particle. By noting where the flashes occurred, the scientists
could determine if the atoms in the gold foil deflected the alpha particles. As with any
good experimental design, the procedure was repeated over and over again to ensure
that all observations were taken and that all results observed were repeated in each
trial. As the number of times an experiment is repeated increases, and the same results
are obtained, the results are easier for scientists to accept.
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Module 3 • The Structure of the Atom