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Lecture 1.2 – Development of Atomic Theory
Lecture 1.2 – Development of Atomic Theory
What was Dalton’s contribution to atomic theory?
I. Dalton’s Model of the Atom
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What was Thomson’s contribution to atomic theory?
I. J. J. Thomson
• First evidence for subatomic particles.
• Made discovery in 1896.
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II. J. J. Thomson’s Experiment
• Cathode-ray tube
II. J.J. Thomson’s Experiment
• Put a battery to the cathode tube and saw a beam form.
• The beam contained subatomic particles.• Positive magnet next to beam = bent
towards magnet.• Negative magnet next to beam = bent
away from magnet.
Table Work – What subatomic particle did he discover?
III. Thomson’s Discovery
• Thomson discovered the electron!• He discovered that atoms must contain
some negatively charged particles called electrons.
• Tells us nothing about the location of the electron.
IV. Plum Pudding Model
• Thompson proposed that electrons were balls of negative charge floating in a sea of positive charges.
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What was Rutherford’s contribution to atomic theory?
I. Ernest Rutherford
• Disproved Plum Pudding Model with Gold Foil Experiment
• Used alpha particles, which are small, positively charged particles.
II. Rutherford’s Experiment
• Shot positively charged alpha particles at gold foil
• Most particles went through foil.
• Small amount of the particles were deflected
• Smaller amount were bounced back
Rutherford noted: “It was almost as if you fired a 15-inch shell into a piece of tissue
paper and it came back and hit you.”
III. What Rutherford’s Data Meant
• Most alpha particles went straight through = atom is mostly empty space.
• Some alpha particles deflected = they came close to other electrons.
• Small amount bounced back = hitting a very small, but very dense positively charged nucleus.
IV. Important Point #1
• The atom is mostly empty space.
V. Important Point #2
• The atom has very small, but very dense nucleus.
• Alpha particles that bounced back were coming into contact with nucleus.
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