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

Rutherford

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Rutherford

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Animation by Raymond Chang – All rights reserved.

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

particlesource

Lead collimator Gold foil

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’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

Geiger-Muller Counter

Speaker gives“click” for

each particle

Window

Particlepath

Argon atoms

Hans Geiger

Geiger Counter

e-

e-e-

e-+ +

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Metal tube(negatively charged)

Ionization of fill gastakes place alongtrack of radiation

Ionizingradiationpath

Window

Atoms or moleculesof fill gas

Central wire electrode(positively charged)

Wilbraham, Staley, Matta, Waterman, Chemistry, 2002, page 857

Free e- are attracted to(+) electrode, completing the circuit and generating a current. The Geiger counter then translates the current reading into a measure of radioactivity.

Speaker gives “click” for

each particle(+)

(-)

What he expected…

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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Rutherford Scattering (cont.)

Rutherford interpreted this result by suggesting that the particles interacted with very small and heavy particles

Particle bounces off of atom?

Particle attracts to atom?

Particle goes through atom?

Particle path is alteredas it passes through atom?

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Case A

Case B

Case C

Case D

Table: hypothetical description of alpha particles

alpha rays don’t diffract

alpha rays deflect towards a negatively charged plate and away from a positively charged plate

alpha rays are deflected only slightly by an electric field; a cathode ray passing through the same field is deflected strongly

... alpha radiation is a stream of particles

... alpha particles have a positive charge

... alpha particles either have much lower charge or much greater mass than electrons

observation hypothesis

(based on properties of alpha radiation)

Copyright © 1997-2005 by Fred Senese

Explanation of Alpha-Scattering Results

Plum-pudding atom

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Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Results of foil experiment if plum-pudding had been correct.

Electrons scatteredthroughout positive

charges

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

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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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Rutherford’sGold-Leaf Experiment

Conclusions:

Atom is mostly empty space

Nucleus has (+) charge

Electrons float around nucleus

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

• Hit moth driving car – no change in car direction

• Hit deer – car changes direction

Alpha particle

Large angle of deflection, must have hit massive object!

moth

deerGold Atom