Transcript History of the Atom

History of the Atom
John Dalton
 British Schoolteacher, 1808
 First to revisit the “atom” and support his
theory experimentally
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Atomic Theory
1. Atom-indivisible solid sphere
 2. All atoms of same element have same mass and
chemical behavior
 3. Atoms of different elements always combine in
fixed number ratios to produce specific compounds.
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William Crooke
 Developed the Crooke’s tube
 AKA Cathode Ray Tube **
 Used to determine the nature of the rays

Shadow (light or electric charge)
JJ Thomson
 The Experiment that proved Light or
Charge***
 Particles positively or negatively charged?
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Used charged plates to test the direction of the
cathode ray. ***
The rays repelled the negative plate.
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What does that mean?
JJ’s Model of the Atom
 The particles are negative! Called Electrons.
 Further experiments using Hydrogen gas lead
to discovery of Protons
 Model
Ernest Rutherford
 New Zealand Chemist,
1909
 Famous Gold Foil
Experiment—Testing
Thomson’s model of
the atom***
 movie
Details
 Alpha Particle
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2 protons & 2 neutrons
4 times the mass of
Hydrogen
Overall positively charged
 Results
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There’s something small
and dense in the center
James Chadwick
 Experiment
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Alpha source shoots alpha rays at Beryllium
(Beryllium - Be – atomic number = 4)
Beryllium rays are shot at wax
Wax ray is formed
Charged plates are presented and wax ray gets
deflected toward negative plate
Chadwick
 Why didn’t the Beryllium ray get deflected!?
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The ray was neutral and unaffected by the charged plates
 Why was the wax ray deflected?
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Because it was repelled by the positive plate (it must
have carried a positive charge)
 Why was a wax ray created in the first place?
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Elastic collisions – total kinetic energy is conserved
This also means protons and neutrons have the same
mass
Chemistry Humor
A neutron walks into a bar and asks the
bartender “how much for a drink?”
The bartender replies,
“for you, NO CHARGE”