the atom - linville the atom.pdf · throughout the atom (plum pudding model) rutherford model •...
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The Atom
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Early Atomic Models• Dalton’s model proposed that the atom was the
smallest particle of matter
• Atoms had no internal parts and were indivisible (like billiard balls)
• All matter was composed of atoms, the atoms of each element were somehow different
• Explained chemical compounds, gas laws
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Thomson• Thomson used a beam of
cathode rays in a CRT with an electric field and a magnetic field perpendicular to the direction of beam travel
• Discovered that the rays are streams of particles much smaller than atoms
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• With only the electric field on, the beam was deflected one way
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• With only the magnetic field on, the cathode rays were deflected into a curved path, in the opposite direction
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• when both fields were on, and the forces balanced, the cathode rays were not deflected
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• Thomson determined the charge to mass ratio of the cathode rays using the curvature of the beam caused by only the magnetic field
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mq
rBv
qBr
mv
Bqvr
mvFF mc
=
=
=
=
⊥
⊥
→
⊥
→→
||
||||2
Everything on the left side could be measured
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• Cathodes made from different metals gave the same q/m ratio, so cathode rays were part of all atoms
• Thomson showed that they had a negative charge and that they had mass, so had to be particles coming from the cathode material
• since the electron was negative, but atoms neutral, there had to be positive charges in atoms
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Thomson Model
• The positive fluid fills the atom while the electrons were embedded throughout the atom (plum pudding model)
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Rutherford Model• 1911 Rutherford,
Marsden and Geiger performed experiments to test the Thomson model
• alpha particles from radioactive sources were directed at thin gold foils (about 100 atoms thick)
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• they predicted the α
would be deflected by the positive fluid of the Thomson model
• the deflection should be small due to the large mass of the α
and the positive fluid
spread throughout the atom
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• results showed most of the α
particles went straight through but a small number scattered at very large angles (not possible according to Thomson)
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• concluded that most of the atom was empty & most of the mass and all of the positive charge concentrated in a very small region (the nucleus)
• scattering angles indicated the size of the nucleus was about 10-15 to 10-14 m in radius
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• electron moving in a circular path would be accelerating
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Problem 1
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Problem 2
• the model could not explain the light emitted from hot gases