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 Reminder:
› Democritus only stated an atom
was the smallest indivisible particle
of matter
that retained its identity.
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 Dalton,
years later
had increased our
knowledge of the
atom but still his
model would have
been the same as
Democritus.
 1897
-- J.J. Thomson (physicist)
 Thomson
experimented by passing
electricity through gases at low
pressures.
 This
gas was trapped in a vacuum
sealed glass with two electrodes
connected to an energy source.
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
2
CRT  Cathode Ray Tube
electrodes
› Anode (positively charged electrode)
› Cathode (negative charged
electrode)
 When
Thomson passed the energy
through the gas a “glowing beam”
appeared and went from
Cathode (-)  Anode (+)
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
Beam was actually a CATHODE RAY
› Beam of charged particles that travel from
the cathode to the anode.

Thomson then approached the beam
with a magnet.
› This experiment led him to declare that there
must be something smaller than just the
atom.
› Completely shattered Democritus atomic
theory.
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 Evidence
from Thomson’s Experiment
› 1. When bringing the – pole of the
magnet near the ray, the ray
deflected away from the magnetic
field.
› 2. When bringing the + pole of the
magnet near the ray it was
attracted towards the magnetic
field.
› 3. The cathode ray traveled from
cathode to anode.

Cathode Ray Tubes and TV/Computers
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
Conclusions:
› There must be something that makes up
atoms that has a negative charge!!!
› Thomson named this a “corpuscle of electric
charge”.
 Later named the electron

Thomson’s Model
› Plum Pudding Model
› “think chocolate chip cookie”
 1911
– Ernest Rutherford
 Gold
Foil Experiment
› Bombarded a piece of gold foil
with alpha particles (positively
charged particles).
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
Gold Foil Experiment

Observations:
› 1. a large majority of alpha particles passed
directly through the foil.
› 2. few particles were deflected when shot at
the foil.
› 3. rarely, one particle would come back
almost directly at the alpha source
 Conclusions:
› 1. Most of an atom consists of empty space.
› 2. There lies a condensed positive mass in an
element.
 Rutherford
discovered the positively
charged atomic nucleus.
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
Although Rutherford was credited with
the positive nucleus, Eugene Goldstein
was credited with the discovery of
protons.

“Anode beam” – similar to Thomson
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
Mass ratio tests were experimented on
these subatomic particles to tell us:
Subatomic
Particle
Electric
Charge
Actual Mass
Relative
Mass
(amu)
Electron
-1
9.11 x 10-31 kg
0
Proton
+1
1.67 x 10-27 kg
1
 1932
– James Chadwick
 There had to be something else!!
› 1. This particle had to be electrically neutral.
› 2. This particle had to contribute to the atoms
mass.
 Chadwick
would later name this mystery
particle the neutron; which is located in
the nucleus of an atom.
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Subatomic
Particle
Electric
Charge
Actual Mass
Relative
Mass
(amu)
Electron
-1
9.11 x 10-31 kg
0
Proton
+1
1.67 x 10-27 kg
1
Neutron
0
1.67 x 10-27 kg
1
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