The Rutherford Experiment and Hit the Penny
The Thomson Model
With the growing evidence that the atom is made of even smaller particles, in the early 1900s
J. J. Thomson reasoned that because electrons comprise only a very small fraction of the
mass of an atom (they are very small), they probably made up an equally small fraction of
the atom's size. He proposed that the atom consisted of a positive sphere of matter with
electrons in it, as shown. This model became known as the "plum-pudding" model, after the
name of a traditional English dessert.
J. J. Thomson's "plum-pudding" model of the atom.
He pictured the small electrons to be embedded in the atom
much like raisins in a pudding or like seeds in a watermelon.
Ernest Rutherford proved this model wrong.
The Rutherford Experiment
In 1910 Rutherford and his coworkers performed an experiment that led to the downfall of
Thomson's model. Rutherford was studying how alpha-particles were scattered as they
passed through the atoms of thin gold foil. Alpha particles are small but dense particles.
Think of them as tiny hard bullets. He found that most alpha particles went straight through
the gold atoms. He expected this because the Thomson model said most of the atom was
just a “pudding” of positive matter, nothing “hard” for an alpha-particle to hit. However,
Rutherford was in for a surprise!
In Rutherford's own words: “I remember two or three days later my assistant coming to me in
great excitement and saying, "We have been able to get some alpha particles
coming backwards." . . . It was quite the most incredible event that has ever Most alpha
particles go
happened to me in my life. It was almost
straight
as if you fired a bullet into a piece of
through
tissue paper and it came back and hit
you.”
Rutherford and his coworkers observed
that almost all of the alpha particles
passed directly through the foil without
deflection. However, a few were
deflected, some even bouncing back in
the direction from which they had come
By 1911 Rutherford was able to explain
these observations; he thought that
Beam of
alpha
particles
Radioactive
source of alpha
particles
A few alpha
particles hit the
nucleus and
bounce back
1. there was a small, dense nucleus with most of the mass of the atom, and all of its
positive charge, (because only a small number of alpha particles bounced back).
2. Most of the total volume of the atom is empty space in which electrons move
around the nucleus. Because in the alpha-particle scattering experiment, most alpha
particles passed directly through the gold atoms because they do not hit the small nucleus;
they merely pass through the empty space of the atom.
Occasionally an alpha particle comes into the close vicinity of a gold nucleus, however. The
repulsion between the highly charged gold nucleus and the alpha particle is strong enough to
cause the alpha particle to bounce back.
The Hit the Penny Simulation of the Rutherford Experiment
In Hit the Penny, we dropped pencils (model alpha particles) onto a model atom consisting of
a piece of paper with a penny on it for a nucleus. Most of the time, we missed the penny, just
as most of Rutherford’s alpha particles missed the nucleus.
a.
Hit the penny:
c.
b
Penny
.
Paper
.
d