1. The Experiment
To test this he designed and experiment directing ‘alpha’ particles toward a thin metal foil.
The foil was coated with a substance that produced flashes when it was hit by an alpha particle.
Source of
a particles
Beam of
Some a particles
are scattered
Most particles pass straight through foil
Thin metal foil
Screen to detect scattered a particles
Alpha particle = two protons and two neutrons
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

2. Appling the Results to the Models
Nuclear atom
Nucleus + -
Alpha particles
Plum-pudding atom

3. Models of the Atom - Greek model (400 B.C.) Dalton’s model (1803)
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg e + + -
Greek model
(400 B.C.)
Dalton’s model
(1803)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
“Models of the Atom”
Description: This slide shows he evolution of the concept of the atom from John Dalton to the present.
Basic Concepts
·         The model of the atom changed over time as more and more evidence about its structure became available.
·         A scientific model differs from a replica (physical model) because it represents a phenomenon that cannot be observed directly.
Teaching Suggestions
Use this slide as a review of the experiments that led up to the present-day view of the atom. Ask students to describe the characteristics of each atomic model and the discoveries that led to its modification. Make sure that students understand that the present-day model shows the most probable location of an electron at a single instant.
Point out that most scientific models and theories go through an evolution similar to that of the atomic model. Modifications often must be made to account for new observations. Discuss why scientific models, such as the atomic models shown here, are useful in helping scientists interpret heir observations.
Questions
Describe the discovery that led scientists to question John Dalton’s model of the atom ad to favor J.J. Thomson’s model.
What experimental findings are the basis for the 1909 model of the atom?
What shortcomings in the atomic model of Ernest Rutherford led to the development of Niels Bohr’s model?
A friend tells you that an electron travels around an atom’s nucleus in much the same way that a planet revolves around the sun. Is this a good model for the present-day view of the atom? Why or why not?
Another friend tells you that the present-day view of an electron’s location in the atom can be likened to a well-used archery target. The target has many holes close to the bull’s-eye and fewer holes farther from the center. The probability that the next arrow will land at a certain distance from the center corresponds to the number of holes at that distance. Is this a good model for the present-day view of the atom? Why or why not?
Suppose that, it the future, an apparatus were developed that could track and record the path of an electron in an atom without disturbing its movement. How might this affect the present-day model of the atom? Explain your answer.
How does developing a model of an atom differ from making a model of an airplane? How are these two kinds of models the same?
Drawing on what you know in various fields of science, write a general statement about the usefulness of scientific models.
Bragg and his father, Sir W.H. Bragg, shared the 1915 Nobel prize in physics for studies of crystals with X-rays.
Charge-cloud model
(present)
Bohr’s model
(1913)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

4. Models of the Atom - Dalton’s model (1803) Greek model (400 B.C.)+ + -
Dalton’s model
(1803)
Greek model
(400 B.C.)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Bohr’s model
(1913)
Charge-cloud model
(present)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1897 J.J. Thomson, a British
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
1911 New Zealander
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1926 Erwin Schrödinger
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
“Models of the Atom”
Description: This slide shows he evolution of the concept of the atom from John Dalton to the present.
Basic Concepts
·         The model of the atom changed over time as more and more evidence about its structure became available.
·         A scientific model differs from a replica (physical model) because it represents a phenomenon that cannot be observed directly.
Teaching Suggestions
Use this slide as a review of the experiments that led up to the present-day view of the atom. Ask students to describe the characteristics of each atomic model and the discoveries that led to its modification. Make sure that students understand that the present-day model shows the most probable location of an electron at a single instant.
Point out that most scientific models and theories go through an evolution similar to that of the atomic model. Modifications often must be made to account for new observations. Discuss why scientific models, such as the atomic models shown here, are useful in helping scientists interpret heir observations.
Questions
Describe the discovery that led scientists to question John Dalton’s model of the atom ad to favor J.J. Thomson’s model.
What experimental findings are the basis for the 1909 model of the atom?
What shortcomings in the atomic model of Ernest Rutherford led to the development of Niels Bohr’s model?
A friend tells you that an electron travels around an atom’s nucleus in much the same way that a planet revolves around the sun. Is this a good model for the present-day view of the atom? Why or why not?
Another friend tells you that the present-day view of an electron’s location in the atom can be likened to a well-used archery target. The target has many holes close to the bull’s-eye and fewer holes farther from the center. The probability that the next arrow will land at a certain distance from the center corresponds to the number of holes at that distance. Is this a good model for the present-day view of the atom? Why or why not?
Suppose that, it the future, an apparatus were developed that could track and record the path of an electron in an atom without disturbing its movement. How might this affect the present-day model of the atom? Explain your answer.
How does developing a model of an atom differ from making a model of an airplane? How are these two kinds of models the same?
Drawing on what you know in various fields of science, write a general statement about the usefulness of scientific models.
Timeline: Wysession, Frank, Yancopoulos Physical Science Concepts in Action, Prentice Hall/Pearson, 2004 pg 114
1924 Frenchman Louis
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
1932 James
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

5. Bohr Model
Neils Bohr
Planetary
model
After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus.

6. Bohr’s contributions to the understanding of atomic structure:
1. Electrons can occupy only certain regions of space,
called orbits.
2. Orbits closer to the nucleus are more stable —
they are at lower energy levels.
3. Electrons can move from one orbit to another by absorbing or emitting energy, giving rise to characteristic spectra.
• Bohr’s model could not explain
the spectra of atoms heavier
than hydrogen.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.