Atomic Theory Timeline

1
Atomic Theory Timeline

• Compiled by Mrs. Johannesson in San Antonio ISD

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Democritus (400 B.C.)

• proposed that matter was composed of tiny
  indivisible particles called atomos
• not scientifically tested
• "Nothing exists but atoms and empty space
  everything else is opinion."

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Democritus (400 B.C.)

• rejected by Aristotle and others who believed
  that matter could be endlessly divided

4
Alchemy (next 2000 years)
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Alchemy (next 2000 years)

• fierce gray wolfsubject to the sway of warlike
  Mars
• antimony is found mixed with iron
• offspring of the ancient Saturn
• antimony is derived from lead

• Cast to him the body of the King, and when he
  has devoured itin a great firethe King will be
  liberated
• antimony is alloyed with gold and heated in order
  to purify the gold

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John Dalton (1807)

• British Schoolteacher
• based his theory on others experimental data
• Billiard Ball Model
• atom is a uniform, solid sphere

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John Dalton
Daltons Four Postulates
1. Elements are composed of small indivisible
particles called atoms. 2. Atoms of the same
element are identical. Atoms of different
elements are different. 3. Atoms of different
elements combine together in simple proportions
to create a compound. 4. In a chemical reaction,
atoms are rearranged, but not changed.
8
Erwin Schrödinger (1926)

• Quantum mechanics
• electrons can only exist in specified energy
  states
• Electron cloud model
• orbital region around the nucleus where e- are
  likely to be found

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Erwin Schrödinger (1926)

• Electron Cloud Model (orbital)
• dots represent probability of finding an e-

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James Chadwick (1932)

• Discovered neutrons
• neutral particles in the nucleus of an atom
• Joliot-Curie Experiment
• based his theory on their experimental evidence

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James Chadwick (1932)
Joliot-Curie Experiment
Be
Energy values werent consistent.
12
James Chadwick (1932)
Chadwicks Explanation
Be

• Energy values were consistent.
• Also accounted for extra mass in the nucleus.

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James Chadwick (1932)

• Neutron Model