history of the atom. john dalton british schoolteacher, 1808 first to revisit the “atom” and...
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History of the Atom
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John Dalton
British Schoolteacher, 1808 First to revisit the “atom” and support his
theory experimentally Atomic Theory
1. Atom-indivisible solid sphere2. All atoms of same element have same mass and
chemical behavior3. 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)
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JJ Thomson
The Experiment that proved Light or Charge**
Particles positively or negatively charged? Used charged plates to test the direction of the
cathode ray. *** The rays repelled the negative plate.
What does that mean?
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JJ’s Model of the Atom
The particles are negative! Called Electrons. Further experiments using Hydrogen gas
lead to discovery of Protons Model
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Ernest Rutherford
New Zealand Chemist, 1909
Famous Gold Foil Experiment—Testing Thomson’s model of the atom***
movie
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Details
Alpha Particle 2 protons & 2 neutrons 4 times the mass of
Hydrogen Overall positively charged
Results There’s something small
and dense in the center
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James Chadwick
Experiment 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
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Chadwick
Why didn’t the Beryllium ray get deflected!? The ray was neutral and unaffected by the charged plates
Why was the wax ray deflected? 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?
Elastic collisions – total kinetic energy is conserved This also means protons and neutrons have the same
mass
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Chemistry Humor
A neutron walks into a bar and asks the bartender “how much for a drink?”
The bartender replies, “for you, NO CHARGE”