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J.J. Thomson • He proved that atoms of any element can be made to emit tiny negative particles. • From this he concluded that ALL atoms must contain these negative particles. • He knew that atoms did not have a net negative charge and so there must be balancing the negative charge. J.J. Thomson

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Page 1: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

J.J. Thomson

• He proved that atoms of any element can be made to emit tiny negative particles.

• From this he concluded that ALL atoms must contain these negative particles.

• He knew that atoms did not have a net negative charge and so there must be balancing the negative charge.

J.J. Thomson

http://en.wikipedia.org/wiki/J._J._Thomson
Page 2: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

William Thomson (Lord Kelvin)

• In 1910 proposed the Plum Pudding model– Negative electrons

were embedded into a positively charged spherical cloud.

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

Spherical cloud ofPositive charge

Electrons

http://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin
Page 3: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

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

PAPER

Rutherford

PAPER

Animation by Raymond Chang – All rights reserved.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
http://en.wikipedia.org/wiki/Ernest_Rutherford
Page 4: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

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

Page 5: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

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.

http://www.nobel.se/laureates/chemistry-1908.html
Page 6: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

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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Page 7: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

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

http://antoine.frostburg.edu/chem/senese/index.shtml
Page 8: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

Explanation of Alpha-Scattering Results

Plum-pudding atom

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Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Page 9: J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these

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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