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The History ofAtomic Theory
(Selected ideas from Ch. 3)
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2500 years ago, Greek philosophers
believed that all matter is made of...
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-Earth -Air -Water -Fire
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• Greek philosopher
• 460-370 B.C
• First to propose that matter was not infinitely divisible
Democritus
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• English chemist & physicist
• 1766-1844
• Considered to be the father of our modern atomic theory
John Dalton
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1. Proposed that matter could be explained in terms of “atoms”
from the Greek word “atomos,” meaning indivisible.
Dalton’s Atomic Theory (1803)
Page 64
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2. Atoms of a given element are identical in size, mass, and other properties.
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3. Atoms cannot be subdivided, created, or destroyed.
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4. Atoms combine in simple whole-number ratios to form chemical compounds.
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5. In chemical reactions, atoms recombine to form new substances.
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Although Dalton’s theory was very successful,
the question remained:
Are atoms really indivisible?
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In 1898 a scientist named J.J. Thomson conducted
experiments using a Crooke’s tube
(see figure 2.1 on p68)
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• all elements give off the same negatively charged particles
• He called these particles “cathode ray particles”
Thomson concluded that...
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Today we call these particles...
e l e c t r o n s !
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Thomson devised the short lived "plum-pudding" model of the atom...
But this model was shown to completely wrong by one of the most important experiments of all time, known asThe Rutherford Experiment