1. Ernest Rutherford (1871-1937)
• Learned physics in J.J.
Thomson’ lab.
• Noticed that ‘alpha’
particles were
sometime deflected
by something in the
air.
• Gold-foil experiment

2. Rutherford ‘Scattering’
• In 1909 Rutherford undertook a series of experiments
• He fired (alpha) particles at a very thin sample of gold foil
• According to the Thomson model the particles would only
be slightly deflected
• Rutherford discovered that they were deflected through large
angles and could even be reflected straight back to the source
Lead collimator
Gold foil
particle
source

3. Rutherford’s Apparatus
beam of alpha particles
radioactive
substance
fluorescent screen
circular - ZnS coated
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

4. What he expected…
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5. Because he thought the mass was
evenly distributed in the atom.
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6. Because, he thought
the mass was evenly - -
distributed in the atom
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7. What he got…

8. Density and the Atom
• Since most of the particles went through, the
atom was mostly empty.
• Because the alpha rays were deflected so
much, the positive pieces it was striking were
heavy.
• Small volume and big mass = big density
• This small dense positive area is the nucleus
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9. The Rutherford Atom
n+
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

10. Niels Bohr
• In the Bohr Model (1913)
the neutrons and protons
occupy a dense central
region called the
nucleus, and the electrons
orbit the nucleus much
like planets orbiting the
Sun.
• They are not confined to a
planar orbit like the
planets are.

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