Download Lecture 1.2 - Atomic Theory Scientists

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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