the history of atomic theory - mrs. rhee science · dalton’s atomic theory (1803) 1. matter could...
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The History of Atomic Theory
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Democritus
• Greek philosopher
• 460-370 B.C.
• First to propose that matter was not infinitely divisible
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John Dalton
• English chemist and physicist
• 1766-1844
• Considered to be the father of our modern atomic theory
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Dalton’s Atomic Theory (1803)1. Matter could be explained in terms of
“atoms” (from the Greek word “atomos,” meaning indivisible).
2. Atoms of a given element are identical in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms combine in simple whole-number ratios to form chemical compounds.
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, J.J. Thomson conducted experiments using a Crooke’s tube
Diagram not
necessary
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Thomson concluded that all elements give off the same negatively charged particles (“cathode ray particles”).
Today we call these particles…
Electrons!!!
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J.J. Thomson’s Plum Pudding ModelEvenly distributed positive charge
Electrons
But this model was shown to be completely wrong by one of the most important experiments of all time…
Draw this diagram!
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The Rutherford Experiment (1911)
Draw this diagram!
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The Discovery of the Nucleus
Draw this diagram!
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The Rutherford Experiment
Atoms… matter… everything is composed almost entirely of empty space!
99.9999999999999% nothing!!!
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The Problem
If electrons are accelerating in orbit around the nucleus, they must lose energy…
So what keeps electrons from crashing into the nucleus???