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Rutherford’s Atomic Model
Ck12 Science
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Printed: December 3, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Rutherford’s Atomic Model
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Rutherford’s Atomic Model
• Describe Rutherford’s gold foil experiment.
• Describe the nuclear model of the atom.
How much space do bricks occupy?
As we look at the world around us, it looks pretty solid. We hit a wall with our hand and the hand stops –it does
not (normally) go through the wall. We think of matter as occupying space. But there is a lot of empty space in
matter. In fact, most of the matter is empty space.
The Gold Foil Experiment
In 1911, Rutherford and coworkers Hans Geiger and Ernest Marsden initiated a series of groundbreaking experiments that would completely change the accepted model of the atom. They bombarded very thin sheets of gold
foil with fast moving alpha particles. Alpha particles, a type of natural radioactive particle, are positively charged
particles with a mass about four times that of a hydrogen atom.
According to the accepted atomic model, in which an atom’s mass and charge are uniformly distributed throughout
the atom, the scientists expected that all of the alpha particles would pass through the gold foil with only a slight
deflection or none at all. Surprisingly, while most of the alpha particles were indeed undeflected, a very small
percentage (about 1 in 8000 particles) bounced off the gold foil at very large angles. Some were even redirected
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FIGURE 1.1
(A) The experimental setup for Rutherford’s gold foil experiment: A radioactive
element that emitted alpha particles was
directed toward a thin sheet of gold foil
that was surrounded by a screen which
would allow detection of the deflected particles.
(B) According to the plum pud-
ding model (top) all of the alpha particles
should have passed through the gold foil
with little or no deflection.
Rutherford
found that a small percentage of alpha
particles were deflected at large angles,
which could be explained by an atom with
a very small, dense, positively-charged
nucleus at its center (bottom).
back toward the source. No prior knowledge had prepared them for this discovery. In a famous quote, Rutherford
exclaimed that it was “as if you had fired a 15-inch [artillery] shell at a piece of tissue paper and it came back and
hit you.”
Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the
vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space.
In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the
atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in
a very small space in the atom’s interior, which he called the nucleus. The nucleus is the tiny, dense, central core of
the atom and is composed of protons and neutrons.
Rutherford’s atomic model became known as the nuclear model. In the nuclear atom, the protons and neutrons,
which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. The electrons
are distributed around the nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small
the nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional
football stadium, the nucleus would be about the size of a marble.
Rutherford’s model proved to be an important step towards a full understanding of the atom. However, it did not
completely address the nature of the electrons and the way in which they occupied the vast space around the nucleus.
It was not until some years later that a full understanding of the electron was achieved. This proved to be the key to
understanding the chemical properties of elements.
Watch a video that explains the gold foil experiment:
http://www.youtube.com/watch?v=XBqHkraf8iE
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/59676
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Chapter 1. Rutherford’s Atomic Model
Summary
• Bombardment of gold foil with alpha particles showed that some particles were deflected.
• The nuclear model of the atom consists of a small and dense positively charged interior surrounded by a cloud
of electrons.
Practice
Questions
Use the link below to answer the following questions:
http://www.icbse.com/topics/rutherfords-model-atom
1.
2.
3.
4.
How thick was the gold foil?
What alpha source did he use?
How many were deflected straight back?
What was one drawback of Rutherford’s theory?
Review
Questions
1.
2.
3.
4.
5.
When did Rutherford and coworkers carry out their research?
What is an alpha particle?
How did Rutherford explain the observation that most alpha particles went straight through the gold foil?
What did he say about the particles that were deflected?
Describe Rutherford’s nuclear model.
• alpha particle: A type of natural radioactive particle, are positively charged particles with a mass about four
times that of a hydrogen atom.
• nuclear model: The nuclear model of the atom consists of a small and dense positively charged interior
surrounded by a cloud of electrons.
References
1. User:Man vyi/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Brick_wall_old.jpg .
2. (A) CK-12 Foundation - Christopher Auyeung; (B) User:Fastfission/Wikimedia Commons. (B) http://commo
ns.wikimedia.org/wiki/File:Rutherford_gold_foil_experiment_results.svg .
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