Rutherford’s Gold Foil Experiment

Rutherford’s Apparatus

beam of alpha particles

radioactive substance

gold foil

circular ZnS - coated

fluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.

Rutherford ‘Scattering’• 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

particlesource

Lead collimator Gold foil

Rutherford’s Apparatus

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

beam of alpha particles

radioactive substance

fluorescent screencircular - ZnS coated

gold foil

What He Expected

• The alpha particles to pass through without changing direction (very much)

• Because the positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

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What he expected…

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Because, he thought the mass was evenly distributed in the atom

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What he got…richochetingalpha particles

The Predicted Result:

expected path

expected marks on screen

mark onscreen

likely alphaparticle path

Observed Result:

Interpreting the Observed Deflections

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

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gold foil

deflected particle

undeflected particles

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.beam ofalpha particles

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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

nucleusnucleus

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Explanation of Alpha-Scattering Results

Plum-pudding atom

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Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Summary of Results

• Discovered the nucleus of the atom.– Positive dense core of the atom (later found to

be composed of protons and neutrons)

• The atom is mostly empty space where the electrons orbit the nucleus.

• The nuclear model

• https://www.youtube.com/watch?v=XBqHkraf8iE